You might want to give
Kurozuka a try; it's a vampire romance that extends across centuries. It's beautifully animated by Madhouse with an intriguing, though sometimes confusing and always gory plot.
The principal story line in
Kure-nai is "disturbing," though not "dark" in the supernatural horror sense. It's definitely worth trying anyway if you've not seen it.
Looking back over the suggestions I'll add a second for
Narutaru. I don't know if you watched it, or how many episodes you watched, but if you sampled just the first episode, you need to give it another try. Episode one of
Narutaru is the most deceptive opening episode I've ever seen.