Blood Hunter is unofficial third-party content by Matt Mercer, but he and Critical Role have an advertising partnership with D&D Beyond.
Nothing makes the Blood Hunter class inherently special as compared to any other homebrew class published on DMsGuild or D&D Beyond. It's not any more official or AL-legal than any other homebrew class.
The only reason it has a special placement/presentation on D&D Beyond (DDB) is that Matt Mercer and Critical Role have an advertising partnership with DDB. Apparently, as part of that partnership, DDB has agreed to provide special listings for Mercer's homebrew Blood Hunter class and his Gunslinger fighter subclass. (The same items appear on DMsGuild: Blood Hunter,1 and Gunslinger.)
DDB's character creator also lists "Critical Role Content" separately from other "Homebrew Content", presumably for the same reason.
This changelog on the D&D Beyond forum discusses the partnership:
Critical Role Partnership
We are also very happy to finally share that we have entered a long-term partnership with Critical Role!
As part of the sponsorship, we will be adding some of the custom content used at the table in the show, starting with the blood hunter class by Matt Mercer.
You can also see in the screen capture above that a new Critical Role Content category has been added. To enable CR content for your character, toggle that category "on" and you will see the blood hunter as a new option in the builder. You can also see the blood hunter under the 'Additional > Classes' section in the main navigation and classes page.
We plan to incorporate more Critical Role content over the course of the partnership, so stay tuned for that here. Also, if you aren't already a loyal viewer, now is an excellent time to jump in and experience a brand new campaign with the cast of Critical Role! Check out the first episode here.
A Twitter user also asked about the partnership and the officialness of the Blood Hunter class:
Twitter user: The Blood Hunter is listed as an additional class on D&D Beyond, under official classes, was it made official?
Jeremy Crawford: The only official classes in D&D are in the Player's Handbook. For a new class to become official in the future, it must appear in a D&D book or must be announced as official by the D&D team.
D&D Beyond: The Blood Hunter is actually categorized under 'Additional Classes' everywhere it appears in DDB. It is specifically not under 'Official Classes' and is not considered official.
Matt Mercer: This. :)
In short: it's not official. It's basically "homebrew"/third-party content, just like anyone else's, except DDB has given it a special placement/designation because of their partnership with Critical Role.
This will not change with the release of the Explorer's Guide to Wildemount book in March 2020. Though it is the first official Wizards of the Coast book about Matt Mercer's setting of Exandria (and the continent of Wildemount within it), it doesn't affect the status of any other content by Mercer.
According to the product listing on critrole.com, the book will contain several new official character options, including 3 subclasses (the Echo Knight for fighters, and the Chronurgist and Graviturgist for wizards) and a number of new spells from Mercer's new dunamancy type of magic, among other new content. However, it will not reprint the content Mercer published on DMsGuild or in Green Ronin's Tal'Dorei Campaign Setting, or make that content any more official.
1 Mercer did release an updated/rebalanced version of his Blood Hunter class in January 2020, but the older version is still available as a separate listing on DMsGuild. He also mentions in this Twitter thread that the version on D&D Beyond will be updated as well.)