It's complicated, and original sources are rare.
What Source material?
After the callout of the existence of the medieval Braunstein and the Barony of Braunstein appearing on the map of the Great Kingdom of ca. 1971 by Gygax, the source material is, besides the artifacts owned by Jon Peterson, somewhat clear: there's at least two magazines or fanzines that could hold answers to the developmental history.
However, Corner of the Table Top isn't currently cataloged on Archive.org, and likewise Domesday isn't either. We know that several of both of these magazines were auctioned in 2012 from the estate of Arneson, yet it is nigh impossible to find even partial scans or snippets that are more than a couple of lines. In other words: the crucial research material is extremely rare.
Domesday - via acaenum
As the question mentioned, Dave Arneson became a member of the Castle & Crusade Society around April 1970. He did receive their newsletter, Domesday, and even wrote for it. In #5, he wrote "Tigers of the Sea", was named the Pursuivant and a Baron in Issue #7, and wrote a description of Lincolnshire for #12.
But the article we are actually interested in can be found in the last distributed issue, #13 of July 1972. This issue contained, so the index, an essay by Arneson called Facts About Black Moor, including a map of Castle Blackmoor and its surroundings. This map would resurface in 1977, when Arneson published The First Fantasy Campaign, slightly altered. The lettering at the edge of the map indicates its wargaming history.
Corner of the Table Top - a trail that kept on giving!
Dave Arneson's own Fanzine might hold more, as the aforementioned auction page tells:
The Corner of the Table Top is the self-published newsletters of Dave’s own campaigns, including the historic announcement of the exact date for his first “medevil BRAUNSTIEN” and Blackmoor campaign details.
Due to the extreme scarcity of this material in digital form, the hunt went on from here. So...
A trail down the rabbit hole
1 - Secrets Of Blackmoor
There's ore! After some hunting I could find some tiny excerpts on the internet: secretsofblackmoor had an article in 2021, commemorating the 50th anniversary of The Blackmoor Bunch on April 17th.
Including the original announcement cited from Jon Peterson, the page contains two excerpts from The Corner of the Table Top:
Vol 3. No 4 - Editor: David L. Arneson - Club News
- There will be a medieval "BRAUNSTIEN" April 17, 1971, at the home of Dave Arneson from 1300 hrs to 2400 hrs with refreshments being available on the usual basis. Players may come at any time and any number are welcome to attend what should prove to be an exciting time. It will feature mythical creatures and a Poker game under the Troll's bridge between sunup and sundown.
Vol III No. 5 - Editor: David L Arneson
On Saturday May 22, 1071 a Brown Stien-type game set in the Middle ages will be held at Dave Arneson's home after the Napoleonic Campaign meeting is completed. All those attending the Napoleonic meeting are invited to stay for this game.
There must have been more games. But in general, details about the Braunstein game in Blackmoor are scarce. We learn from the secretsofblackmoor page that Bob Meyer did sit out several sessions after his character was killed in the very first game on April 17, but he also became the first to become Level 20 ever. Arneson would sculpt a dragon called Gertie for the game. But...
There's more. As the games ran, Arneson would experiment with character matrices to depict individual characters. Like on May 22nd 1971, Arneson created a character matrix for the Kingdom of Spain, probably for the Napoleonic wargame meeting that was happening before the Braunstein of that day. Dan Nicholson was handed it or a copy of it.
2 - D H Boggs
And then there's even more! Following the Link at the end of the description of those early Braunsteins, we find D H Bogg's essay "The First PC to Die, Ever", likewise from April 2021 to commemorate the 50th anniversary.
Our earliest record tied conclusively to Blackmoor is the Northern Marches map the Areneson mailed to Rob Kuntz in March of 1971 - just weeks prior to the Troll Bridge poker game.
So Braunstein existed a little before the game in the head of Arneson. That ties up well with Gygax map of The Great Kingdom, which put Blackmoor and other counties and duchies at the northern edge. But the letter also predates Arneson having a copy of Chainmail in his hands:
What we are seeing with the letter and map to Kuntz is Arneson's initial planning, at a time before he has a copy of CHAINMAIL with its Fantasy Supplement.
In fact, the map by Gygax actually reflects a change that Arneson made due to players leaving his initial planned campaign: the Egg of Coot and other "Infamous Characters" of Blackmoor were modeled after players that would not play in the pre-publication Blackmoor. It is to be assumed that the map made by Gygax only came into existence reflecting at least the first or first two Braunstein games.
Only in or after the May issue of 1971, so Boggs, the Corner of the Table Top would call those games Blackmoor, indicating that this would be the earliest the map could be made. We also learn that a lot of the group that took part in April wasn't part in the May game - when Bob Meyer was slain by the bridge troll.
What game, if any, may have taken place on May 22 isn't clear, possibly it was the first Castle/Dungeon game inspired by a monter movie binge that Arneson reported so often
We also learn from here that at least one of the Blackmoor-Braunstein games was some kind of rescue adventure, which presented Dave himself getting lost in an Icelandic cave and ending in Blackmoor. It's however unclear if that was the May or a later one. The only fact that remains is Bob Meyer wasn't there and has the honor of being the first to ever be slain in an RPG. And he'd take over running Blackmoor after Arneson passed.
3 - The Comeback Inn
There's More! Reading the article brought me to the The Comeback Inn / Blackmoor forum and indirectly to its owner, Havard's Blackmoor Blog. Havard has run the forum since 2002 and might be one of the biggest experts on Arneson's Blackmoor besides some other members of his forum.
Was this [April 1971 meeting] actually the first Blackmoor game? That is where fans and experts disagree. Most accept that Dave Arneson ran similar games before this time even if they were not announced in the fanzine. Several of his players have said that they remember games at least as early as 1970. Jon Peterson has referred to these games as Blackmoor "precursor games" which leads us to a discussion of definitions, which we can get back to in a future blog article.
But that's not all that the Blackmoor Forum can give - the accompanying thread to the blog post points back to other articles. Dustin Clingman (TheMystic), a friend of Dave Arneson who was working with him on republishing Blackmoor for 3rd Edition, has a Q&A there and answered some questions about the publication history, and the stories Dave told. In general, the forum is a treasure trove.
Blackmoor Gazette and Rumormonger
There's More! Indeed, let's go back a few years. In 2017, D H Boggs discussed Blackmoor as a CHAINMAIL Campaign, indicating that in large parts it actually wasn't. And in it, he points to a different publication entirely:
Our primary documentary sources for the Blackmoor campaign 1971-1975 are The First Fantasy Campaign(TM), The Corner of the Table Newsletter, and the Blackmoor Rumormonger and Gazette newsletter.
Another primary source? Yes. Blackmoor Gazette and Rumormonger #1 is another of Jon Peterson's artifacts. It dates to about 1971, and is described by Peterson as follows:
Like "Points of Interest," [the Domesday #13 article] the BMG&R does not tell us a great deal about the system of Blackmoor, but it does give significant insight into the setting and the state of the campaign at the time of its publication.
It contains some pointers about what kind of adventures happened (mainly: Dragons and forces of evil) but dungeneering wasn't a thing in this issue. In fact, D H Boggs pointed out several times in his own blog how much of the early Arneson games were focused on above-ground adventures, and that only changed later on. Though as Jon Peterson noted, issue 2 of the Gazette was discussing quite some dungeon adventure, and the Gazette can probably serve as the best indicator for the campaign's development. It is, however, probably even more rare than even Corner of the Table Top.
Blackmoor wasn't just Chainmail
But back to the other discussion of Blackmoor as a Chainmail game. As Boggs discussed, there'd be evidence of Blackmoor using some of Chainmail's rules. But also that Arneson diverged a lot from Chainmail in some aspects (e.g. multiple hit points for characters) and that Arneson and his club were heavily invested in Strategos. In fact, while some items might point to Chainmail origins, like magic swords, Arneson diverged a lot from Chainmail, especially when it came to magic.
Other materials pointed to Strategos material that was in use in the Twin Cities. For example, Don't Give up the Ship contained an adaptation of Strategos N's Morale system.
D H Boggs finally comes to the conclusion, that Chainmail was used in the early Blackmoor games more as a supplement than an exclusive ruleset - and that most of the gaming was done with Strategos rules or its variants. Arnesons Braunsteins wasn't a "Chainmail Campaign" but a Braunstein that adapted Chainmail as it was convenient.
An open end - inferences and research pointers
But... I know, there are still questions open. Like, we know little to nothing about what exactly came into being about Blackmoor between the April and May meeting and Domesday 13. We know even less about material that was written between Domesday 13 and Supplement II Blackmoor.
But we can infer a lot: We learned earlier that Arneson would invent stuff and make it up, binge on monster movies, have an open house policy for most people, a roulette of games run in his cellar, and most of all, captivate the minds of people by how he told and ran his Braunsteins. He'd even got poor Bob Meyer to re-join that game, allegedly because of how much fun it was for the players. And then allow him to succeed to become the most powerful wizard of Braunstein.
We also know that we should keep our eyes peeled for any copies of Corner of the Table Top, checking if the later issues of Volume 3 or later do indeed contain the promised campaign notes for Blackmoor, and whatever issues of the Blackmoor Gazette and Rumormonger happen to survive. And if they do, whoever gets his hands on them surely has the real treasure trove on this topic. Or you happen to be Jon Peterson and discuss this part in your book Playing at the World, in Section 1.10.
That's about It for today's deep delve into the early history of pre-D&D.