Into the rose garden
Madk
The summer hikaru died
MADK
The warehouse
The red mansion
The pawn's revenge
Interview with a Murderer
Just when I thought I came here to die
Saha (Lee Uin) - a lot of people have issues with this one because it purposely leaves a lot open to interpretation. I personally loved it.
Living Dead (ASADA Nemui)
Monster and the Beast
OUROBOROS
Itou-san (KURAKA Sui)
Depending how dark/horror based you want to go Asada Nemui, Nishin Masumi and Harada have put out a lot of dark, horrifying and weird stuff.
It'd like to read a dark BL manga, something interesting! I hope you can help me! I'm also cool with horror genre.