I think it's just an overlap of properties.
Gravity is the force produced by an object that pulls stuff towards it.
BB's Darkness is a little more "wide" than that. It's the ability to absorb everything. And when I say absorb, we can use pull, engulf, consume, envelop, etc, etc.
So while Fuji's ability is more "restricted" to the very nature of "gravity" itself, BB's power is more..."magical" than actually practical. I don't exclude the possibility of the Yami Yami no Mi being able to pull meteros from space just like Fuji's. BUt that would take a level os mastery of the fruit that I don't think BB has.