I will be starting a new campaign soon, and have been trying to plan ahead on how to build a fairy blaster warlock. My premise is pretty simple, toss out a concentration spell for either damage or battlefield control, then fly up into the air (if possible), and start blasting away with Eldritch Blast, while using Grasp of Hadar to lift enemies into the air (causing fall damage and knocking them prone). The only hangup I have on the build so far is the fairly low AC  of 14 I would have at level 12 (that's as far as I have the build planned out, as I doubt this particular campaign will make it much past that).

Will the low AC be a problem for a ranged blaster like this? Even using Mage Armor would only help with +1AC. Should I take a feat in order to be able to get a shield? Or should I forgo the higher stats in CHA and CON to boost my DEX some more?

BTW, this particular DM has disallowed multiclassing for this campaign, and medium armor would disallow the racial flight ability.

If you're properly positioning your character, there shouldn't be a problem no matter how low your AC is. Now, the issue I see with your grasp of hadar plan is that it only works once per turn, you probably won't get much juice out of the fall damage portion, since you're aiming to dealing 1d6 per turn ontop of everything else being done with your EBs. Plus, knocking things prone isn't always advantageous. I think if you're concerned about positioning, you'd be better off with eldritch spear to increase your EB's range and allow you to stay in a better position. For example, you could set up one turn with dimension door 300 ft into the air, stay there with flight, then the following turns rain your EBs on the battlefield with impunity. You could also go with repelling blast, which is always handy-dandy and pairs well with repositioning for superior strats!

Just, whatever you do, ensure agonizing blast is your first invocation. 

Quote from cgarciao >>

If you're properly positioning your character, there shouldn't be a problem no matter how low your AC is. Now, the issue I see with your grasp of hadar plan is that it only works once per turn, you probably won't get much juice out of the fall damage portion, since you're aiming to dealing 1d6 per turn ontop of everything else being done with your EBs. Plus, knocking things prone isn't always advantageous. I think if you're concerned about positioning, you'd be better off with eldritch spear to increase your EB's range and allow you to stay in a better position. For example, you could set up one turn with dimension door 300 ft into the air, stay there with flight, then the following turns rain your EBs on the battlefield with impunity. You could also go with repelling blast, which is always handy-dandy and pairs well with repositioning for superior strats!

 

Just, whatever you do, ensure agonizing blast is your first invocation. 

 Mostly, the grasp of Hadar was for the fall prone effect, since the other three members of my party are all melee-oriented (fighter, cleric, and melee rogue). I was thinking that knocking one target prone would be a great way to give the melee guys advantage on one target at a time while they gang up on the down baddy and hack away.

And of course, agonizing blast will be one of my first picks. Here is how I was planning my PC at level 12:

https://www.dndbeyond.com/characters/115737845

You’ll have the typical problem that flying characters do. You’re up in the air with no cover. A pretty easy, obvious target for enemies with range, and the prime target for enemies who can fly. And of course anyone who has an effect that can knock you prone will have you taking the fall damage, though that’s not as much dependent on AC. 
Where it really falls apart is indoors where you can’t fly up at all. 
That said, a 14 AC will be ok mostly with three melee types keeping them off of you. You might invest in something like fey touched for misty step to get away if needed. (Maybe you did, I didn’t look at the sheet.) And maybe don’t count on the flying gimmick to be quite as strong as you are thinking it will be. 

At earlier levels, it should be okay, but I would recommend getting the Armor of Shadows Invocation to buff up your AC all the time, because otherwise unless you position yourself perfectly, your going to get put in a lot of bad spots with low AC.

As long as you are in the right place you should be fine without a high AC

You can be fine, but there are unintended drawbacks if your DM tires of your shenanigans.  Archers, ranged attackers and mages to simply dispel your flight would be problems.  There's a 0% chance that I allowed a player to get away with what you propose. Oh, I'd let you do it, but there'd be consequences for what I'd take as extremely cheesy.  There's no way that you would avoid being singled out for extra attention by any intelligent enemies.  Grasp of hadar-ing you right back to allow you to eat your own fall damage is an easy pick.

TL/DR?  Low ac is fine as long as you don't become a frequent target of ranged attacks.  Avoiding being an obvious target is generally pretty key to getting away with low AC.

14 is pretty low but not a death sentence.  From your post I gleaned you have a 14 dex.  With Mage armor you could get to 15.  By level 12 it is reasonable to assume that you may have gotten some items to boost that a little, but likely you will always be on the low side.  

Recommendations to manage the lower AC.
1. Shadow of Moil.  Be close to invisible.  
2. Misty step.  A lot of monsters will grapple you.  Fey touched is a good way to get this spell an extra time per day.
3. Positioning.  Use Repelling blast to keep things off of you and get cover.

Recommendations to improve the AC.
1. Medium armor feat.  Shields are +2 and as a blast you have an open hand.  Also if you are in a long ceiling dungeon put on some medium armor like a breastplate and get another +2.  
2. Cover improves AC.

My warlock is level 8 and also has an AC of 14.  Hasn't been a noticeable issue so far.  A blaster shouldn't be close to the sword-y types, and should be using cover and other positioning to not draw a ton of ranged fire.  Repelling Blast is a great way to ensure things don't swarm you, and group tactics should be a help too.

In a balanced campaign where people aren't grossly overpowered; you won't be good at everything.  You are sacrificing AC for damage capability, which is proper for your build. 

I forgot one long term option.  At 8th or 12th you could take Gift of the Metallic Dragon for a non spell slot way to make a hit a miss.  And you get a 1st level cure wounds once per day (and Cure wounds as a spells known).

No. It’s only a death sentence for hexblade’s, or other martial warlock builds :). Most of the time you’ll engage in spell battles, so saves are more important.

If you're really set on flying in the open: Darkness spell + Devil's Sight Invocation = Advantage to your attack rolls, disadvantage to theirs. 

Mage Armour is equivalent to Studded Leather +1 or Leather +2 magical armour, so it's always worth taking over regular light armour. 

Although, frankly, not flying out in the open and instead using cover and using Hex spell achieves better damage output than your tactic while giving you more protection. You'll be trading +1d6 per turn fall damage (since grasp of hadar only triggers once per turn) and 14 AC, for 1d6 necrotic damage per eldritch blast beam and 16-19 AC. Not factoring Mage Armour.

I mean you can use Hex instead of Darkness while flying - but your low AC will mean you get hit more so the darkness would be necessary to keep yourself safe, and you can't have both. I dunno about you, but I know which one I'd pick.

Quote from Cyb3rM1nd >>

If you're really set on flying in the open: Darkness spell + Devil's Sight Invocation = Advantage to your attack rolls, disadvantage to theirs. 

Mage Armour is equivalent to Studded Leather +1 or Leather +2 magical armour, so it's always worth taking over regular light armour. 

Although, frankly, not flying out in the open and instead using cover and using Hex spell achieves better damage output than your tactic while giving you more protection. You'll be trading +1d6 per turn fall damage (since grasp of hadar only triggers once per turn) and 14 AC, for 1d6 necrotic damage per eldritch blast beam and 16-19 AC. Not factoring Mage Armour.

I mean you can use Hex instead of Darkness while flying - but your low AC will mean you get hit more so the darkness would be necessary to keep yourself safe, and you can't have both. I dunno about you, but I know which one I'd pick.

 That is a fair point. Having never played a warlock before, I hadn't noticed that Hex was a concentration spell. Might have to rethink this build idea.