I have to say that I find the writing in Black Diamond pretty much on point. Obviously, there are some far-fetched things happening in the overall setting, that's true for most VNs. But the way the protagonists interact with each other feels very natural, believable and interesting.
Add to these really nice renders and a slow but steady progression without any time fillers like sandbox, and you have ingredients for a very good visual novel.
I will say that the initial encounter in the train raised all kinds of red flags regarding pure corruption type games, which in most cases are of low quality. And I would definitely say that BD has a significant focus on that genre. But the way it is presented, the slow revelation of all those little secrets other protagonists have, it all feels very thought-through.
Every character has their own problems and goals in life and although the female MC is tied to all of them to some extent, it still could be a world that existed without her.
For the great scene-, dialogue writing and renders alone I strongly recommend this game. It is female protag, it is a slice of life, it is corruption porn which not everyone is crazy about (it's not my favorite genre as well), but it is first and foremost an excellent execution.