So far, my personal experience is building the same kind of character you are speaking about, a swashbuckling-style Warlock. Now, while I don't have all the theorycrafting knowledge these other guys do, I can tell you what I'm planning on and I think it'll work out rather nicely.
Human Warlock, Pact of the Blade, Archfey. First off, the feats I'm planning to pick up are Medium Armor Prof, Warcaster, and Defensive Duelist. If it feels right, I'll pick up Medium Armor Master too. What that does is it gives me incredible survivability without taking away my ability to spellcast in melee combat.
The general idea is to use a Rapier, since I'm a dex build (8str) and a shield in my other arm. That, plus MAP/MAM gives me a pretty high AC. That can get even higher if I use Defensive Duelist. Yes, it takes up my reaction, but it can be used AFTER a confirmed hit. Generally you get told if you're hit or automatically hit (they rolled a 20) so if they auto-hit, you don't waste it.
Combo this with Mirror Image. No concentration, 3 images, and you roll to see if they hit those instead of you. They don't have nearly the same AC I do but that's fine, they're there to take one hit and poof. Now, if I'm not auto-hit, and they don't hit an image, but hit me directly, I can use DD to add my proficiency bonus to my AC and force them to attack THAT number. Since my AC would already be pretty high, I've got a good shot at going over 20 with that boost, which is hard to hit. So it's a pretty safe bet that I'll deflect almost everything coming at me.
This makes me sort of a tank, just not as good as an actual tank, and it requires a lot of feats, hence variant human. What this DOESNT do, is eat up my invocations. I only need to spend 2 on the Pact of the Blade boosts, and those only come at 5th and 12th. This leaves me open for casting False Life at will, so I almost always enter combat with 8 temp HP. That has saved my ass pretty well so far in Horde of the Dragon Queen. Devil's Sight is nice too, seeing through even MAGICAL DARKNESS. BBEG casts Darkness, you're fine, go up, stab him in the face. It doesn't delay you until your cleric(if you even have one) uses Daylight.
Part of the reason I picked Archfey is because I wanted the flavor of it, the other part is all the abilities work best when you're in close. Fey Presence can only hit a 10-foot square adjacent to you, so if they're not near you, it's useless. Fear for 1 turn is nice, and can make low-wis characters like the Big Stupid Fighter turn and run for long enough to give your party breathing room. Misty Escape lets you get the heck out of dodge if you are in trouble, and in melee, you're likely to get in trouble more often than if you're at range. Another reason is Greater Invisibility gets added to your spell list. It can be great for getting a good positioning on the boss, and with the added staying power of the rest of the build, you can actually hold that position well and deal good damage.
Spells will always hit harder than weapons, and some cantrips are more useful or powerful than weapons, that's just the way it is. But it's not really a big nerf to choose to build for melee combat, since you can then do something no Eldritch Blast spec warlock can: hold the line for the party.
So far, I'm only level 4, but I can see this playing out very nicely in the long run. It'll only get more effective the more levels you gain, which is the mark of a good build. Anything truly effective should get better with time(and EXP!).
I hope this helps give you ideas!