I dont think this word means what you think it means.
I dont think the DCEU films are all that dark. They're not happy and bright and full of sunshine and puppies, but "dark"? We're not talking Lovecraft here.
These films are simply taking the fantastical, high-concept characters we love and inserting them into a world closer to our own, where flying aliens who break all the laws of physics don't get global acceptance just because they smile and save a cat from a tree. That's not dark, its just not humorous.
Of the DCEU films, I truly, deeply enjoyed Man of Steel (flaws and all) and Wonder Woman (even if it was too safe). I think Suicide Squad was a solid "C" and I think the ideas they were playing with in BvS were interesting but the title fight soured my stomach to such a degree (Im a huge Superman fan) that I cant forgive it.
The films arent as well put together as the better Marvel movies. They're not the same kind of cheap, empty popcorn entertainment. That doesnt mean the concept they're going for is bad.