So, what exactly do Bluestar's kids know about their dad?
Absolutely nothing.
It's pretty taboo to ask a she-cat (or anyone she-cat adjacent) who sired her kittens, and while Misty, Moss, and Stone may certainly have noticed that no one claiming to be their mother's partner came to visit, it wouldn't have been out of the norm.
They do know the story about their sire- at least, they know it the way Blueflood tells it. Their mother mistook friendship for love and threw herself into a whirl-wind relationship with their sire, but quickly broke it off, and they never saw each other again. Blueflood feels sad that she never loved him the way that he loved her, but she can't regret the time they spent together, because it resulted in her children.
Misty, Moss, and Stone respect that she doesn't want to tell them who their sire is. They don't really need the guy, anyways- they've got an awesome mom and a great community. Sure, it would be nice to meet him some day, but they're also perfectly alright with it if they never do.