RAW is really funky where vision is concerned; it seems that if your friends and enemies can't see in the dark, they'll be rolling without advantage or disadvantage because of Blinded cancelling out on both sides. Having Darkness will be a bit rough on the rest of the party because most spellcasters need to be able to see their target, and if your DM is a little harsh then your ranged attackers will be having lots of trouble as well.

I would definitely suggest getting something made for your Darkness; something like a hooded lantern but that closes completely. The idea is to be able to switch the Darkness on and off without actually having to recast it (10 min duration means you can sometimes keep the same casting between fights) by hooding the lantern as your object interaction. Good for opening up the room for your allies to strike too, if it's a small one. It also allows you to pre-cast your Darkness before the battle begins if it's a predictable one, saving you an action.

Unlike most Hexblades, you won't be casting Hex much so maximizing your attacks isn't quite as important; Booming Blade will be great for you as you can move up to someone, Booming Blade and then move away (can't OA you because they can't see you, PHB p. 195), so they have to trigger it to hit you. It would still be nice to have a resource-less bonus action attack, so you might consider Polearm Master.

As for invocations, you're not going to be able to cast the regular Hex yourself as you'll be concentrating on Darkness so the Warlock hex trigger invocations won't do you much good. I'd pick up Agonizing Blast for a solid ranged attack, or possibly Thirsting Blade if you get to Lock5 before you hit Bard6.

Feat-wise, you NEED War Caster. Regardless of your disadvantage, you will get hit, you will lose your Darkness, and then you will get piled on. I would also greatly suggest Great Weapon Master, even though it doesn't play with the Swords fighting styles, as your always-on Advantage and Cha-SADness will compensate for the accuracy disadvantage and let you hit hard. Elven Accuracy will make that amazing, as well.

I wouldn't go for Find Greater Steed unless you have a good plan for it; it's not that powerful a minion and it doesn't have your Devil's Sight. The only reason I'd go for it is if you have some really good spells to share with it. One thing you might consider (but probably won't work for your concept) is to get Sharpshooter and pick up Swift Quiver with your Lore option, which would make you an arrow-spewing engine of death. Spiritual Weapon is a good idea, and it's possible (check with RAW-heads or your DM) that it might benefit from your Darkness (as it's technically you making a melee spell attack, and you can see). I would recommend picking up Thunderstep, as a matter of utility, and definitely get Shield from Hexblade; you'll be able to cast it with your Bard slots.

Have fun eating the souls of your enemies!