Darkness needs a party that can work with it. The Eversight Ring and Warlock's Devil Sight eldritch invocation both will preserve Advantage and let you shoot in/out/through it. There's also a helm very late in the game. As the other commenter was alluding to, there's a key component here, and it sounds like you haven't picked it up yet.

I tried this build in my first playthrough and ultimately I wasn't thrilled with its performance. Darkness is smaller than the big surface spells (Hunger of Hadar, Spike Growth), and a big part of the reason why those spells are so effective is their size. Put most (or all) of your party in Darkness and it's quite cramped. And not very mobile. I think if I were to try it again, maybe I'd put it in more of a tanky/control build. Combine Darkness with a Ranger's Entangling, maybe.

If you can handle the deviation, getting Shadowheart out of the Trickery domain would be a big boost regardless. I don't think War is way off. There's a mod adding the Death domain, if that sounds any better (though I haven't tried it).