I don't hang out with black people that often. I have a few black close friends, and a few coworker-type people I see very often but don't interact in any deep or meaningful way.
But I think that's beside the point. I didn't take all the people I know and say "out of all the black people I know, not many are polyamorous". Because, how would I know that?
What I did was think of polyamorous events I've been to (events that were not race-specific) and what kind of people attended. And the huge majority of them were caucasian.
So from my small, not very representative perspective, most poly people I interacted with as poly people were white.
Doesn't mean other races aren't poly. Just that they don't go to the same meetings I went to.
I too have no clue what a black club or a black event are.