It can sometimes depends on the genre and what feelings you want to go for.
If it's a horror BL, the genre kinda calls for a downer ending sometimes, if not at least bittersweet. If it's romance comedy, I feel ending it on a lighter note definitely sells it. But again -- if you want feelings opposite of what the genre is, that's also an option. Some people want a happy ending to their horror, and others may want a bittersweet ending to that romance comedy.
For myself, I'm really drawn to endings that are "open book". The character's journey will still continue on after the book is closed, but we've closed it on one chapter in their life. That's always satisfying because as a fan, I can always think of the ways they continued their journey from there on my own.
So, for example -- couple may be moving in together, and that's a big step for their journey. Maybe the story has come a long way from them being friends (or even strangers) and showcase that romance buildup. Or maybe they've just finished an arc that was life-risking, and the important thing now is each other and their own paths. So now they're just settling down with each other, understanding what each of them want from their own lives, and the story ends there.
It's an uplifting note, and leaves room for fan interpretation. It also gives you an option to either continue from that point to just leave it as is. But more so, it's...reassuring. After all the drama they've gone through, and after everything they've gone through together, they're still there for each other. It's comforting in a way, and thus gives the audience that feeling of "whatever else comes next, they can get through it together"
Course, if the BL isn't that soft, there's always those bittersweet endings that still give room for some hope? Maybe the two split apart) -- maybe it wasn't even romantic at all. With that, I wouldn't mind a BL where one (if not both) realize that they shouldn't be together and they go their separate ways (however easily that happens).