Well people.

Mono's got it.

It might not seem so at first, but just keep watching. It'll dawn on you after a while. Don't blame us when you're DEVASTATED.

Can't really think of any anime that fit better than Darker than Black.

This show is impossibly dark. If you asked how much more black it would be, it would be none. None more black.

I reccomend just browsing sonic guro fanart on rule 34 tbqhf.
Shit's dark.
Also NTR.

Aku no Hana would probably count, although I've only actually read the manga and never seen the anime.

Anime doesn't really get all that dark. But Texhnolyze is worth checking out. It's not as gore focused as something like Elfen Lied but has one of the darkest and depressing atmosphere I've seen in anime so far.

You're better off looking for horror manga.

a month later

My brother had one on video years ago called Urotsukidoji (or something similar), that was pretty "spoken of in whispers" around my college (UK meaning of college) in 2001, but I never watched it myself.
There was also an anime prequel to Ichi The Killer in one of the deluxe DVD's, that was pretty screwed up.