Responses
I don't know if this is it since it's not BL but josei, but it sound similar to what you described
https://www.mangago.me/read-manga/lover_of_green_midnight/
beta
It's set in a historical time period.
I vaguely remember the details but I think it was a BL
There's our main character who is a servant at a duke's(not sure?) place, he has a girlfriend too who is also a servant.
The Duke(?) is away and later comes to his mansion and he starts to flirt or something with the MC's girl. And the mc starts to fall for the Duke (not sure). The MC and ML have a conflict i think but then the ML starts pursuing the MC. The MC falls in love with him and they start having a physical relationship. The girlfriend gets to know about this and feels very betrayed.
Only thing i remember is that the Duke has green eyes and the girlfriend paints the Duke with a green (i think it's arsenic) and slowly starts falling sick.