Give me the darkest-themed yaoi manga you've ever read in your life.
Psychological is of course a must.
The bloodier and gorier it gets, the better it is.
From incest to cannibalism, I'll take anything.

Anyone?

I don't normally read a lot of dark stuff, but one I've enjoyed was Sakura-Gari.

Other stuff that may be what you're looking for:
Songs of Self-Abuse
J no Subete, The Inheritance of Aroma and most stuff by NAKAMURA Asumiko

Also in terms of horror/gore, try Kanpai! dj - Unapai and pretty much any of MURAKAMI Maki (村上真紀)'s doujinshi dead

And next time, try using the search bar at the bottom of the 'Yaoi/Yuri' forum (which this thread should be in the first place):

http://www.mangaupdates.com/showtopic.php?tid=41235
http://www.mangaupdates.com/showtopic.php?tid=22527
http://www.mangaupdates.com/showtopic.php?tid=20307
http://www.mangaupdates.com/showtopic.php?tid=25476
http://www.mangaupdates.com/showtopic.php?tid=45662
http://www.mangaupdates.com/showtopic.php?tid=44170

One of the most notable is Zankoku na Kami ga Shihaisuru by HAGIO Moto. It is largely psychological, but there is physical and sexual abuse throughout.

There are three others I am fond of but none are completed. They probably all need new scanlators as Anonymous Delivers seems to have dropped them...

Innocents Shounen Juujigun
Litchi Hikari Club
Bokura no Hikari Club

All are by FURUYA Usamaru

They are more dark and dystopic but do have moments of gore. All three touch upon shounen ai, with some minor sexual activity, but it is not a large focus. The main draw is Furuya's intense art style which is very suitable for dark stories.

Black Sun, but it made me a bit depressed.

Shinkai no Venus is pretty fun, but not as dark.

I don't know if what you are looking for in part overlaps with this: https://www.mangaupdates.com/showtopic.php?tid=45053&page=1#post655433

Which dark ones do you like?

I dont know how dark you want, but for me there are three things in manga I'm not very fond of: incest, threesome & love triangle and OhMyGod groupie by Nitta Youka has all three of those elements. I dont know whats got into me to actually finish reading that manga probably because of Nitta sensei excellent drawing.

After finish reading it I read a lot and I mean A LOT of comedic manga just to clean my brain!

A good novel tells us the truth about its hero; but a bad novel tells us the truth about its author. - G.K Chesterton