We can get more specific about demographics. But it's certainly not any combination of demographics. We usually place specific importance on demographic divides that feature particular conflicts or differences in institutional power.
Do you not think there is a considerable difference in the institutional power of black Europeans in comparison to black Africans throughout the history of colonialism? What about mixed-race people? Should they be excluded due to the differences in institutional power afforded to them under colonialism? Their presence might change the conversations being held. Am I to be counted as white because I pass? Is that not simply colorism? Or are we playing blood quantum games?
But more to the point, of what relevance is this? Just because there are many different places where we could draw a line, doesn't mean a line cannot be drawn somewhere based on people's best efforts.
The point of this is that the premise that "People will discuss the issue differently or more freely in a group of only X" is not particularly compelling in and of itself as a reason to exclude individuals from a part of a public venue on racial criteria.
If the line was drawn at black Africans only, and not allowing black Europeans to participate, what would your reaction be then, do you think? If there was a day for whites only, how would you feel?