
As SL stated, dark romance as a genre has always been a thing. There were even plays way back in history that centered their conflicts around tragic lovers, abuse and a whole lot of other things. It's just that in recent times people have deemed such themes as "unsanitary" and therefore think it hasn't been a norm until the last decade or so.
Also, bl/yaoi started in the east as a form of coping for the women who were severely repressed. Now it's evolved into a more broader audience, but that doesn't take away its history.
Lastly, you'd be shocked at the number of gay men who write or draw these kinds of themes, too. Okane ga Nai is made by a gay man and it's one of the godfathers of toxic yaoi lol geicomi is also predominantly made by gay men and it's full of incest, rape and domestic violence. It's not just women drawing these.
Tbh i did a deep dive into South Korean culture and life over there i wanted a better understanding why the BLs that come out from there feature a lot of rape, abuse, bullying, misery just not nice things happening to the MCs and why the MCs just accept it and move on with the perpetrator and tbh i have a better understanding of why they write BLs like they do. I don’t agree with it but i definitely understand why.