I'm not driven toward dark stories more than lighter ones but I like when they're well done. I was recently searching for some serious manga/anime and although I still have to read Lone Wolf and Cub, Akira and other classics, I'm a bit too lazy to read. I've watched Black Lagoon for that reason, have you seen it ?
Anyway, I think I'm gonna try Psycho Pass right now, I'll tell you what I think of it later.
Have you tried Monster or any other Noaki Urasawa's manga ? If not then i highly highly recommend that you try the Monster anime and see if you like it .