For dark-ish stuff I'd always recommend
D.Gray-man, it's really amazing; the only problem is that it's 103 episodes long, and that's quite a turn-off for most people. There's also
Jigoku Shoujo, which is both supernatural and very, very dark.
Kinda makes you lose faith in humanity, even. It's episodic, though, so it depends on if you like episodic anime; but if you try it out and like it, then you'll be happy to know it also has two sequels :D
Shiki is a vampire/horror/splatter series, and it's very good once you get past the strange art style, but only if you really like the macabre genre, there's lots of blood so it's not for people with a weak stomach. And for dark nonsensical humor, try
Sayonara Zetsubou Sensei and its numerous sequels c:
As for something completely different, both
07-Ghost and
Pandora Hearts are series that you might enjoy since you liked Vampire Knight, Kuroshitsuji and Ouran :D