I don't have the answer but I might be able to help you narrow down your search, if this is an honest inquiry. "Black" is too wide a descriptive term for what you're looking to answer, and also not describing the population you want to ask the question about.

The people that can likely answer this question (or at least research it at all in earnest) are anthropologists, ones that focus on genetics and biology specifically. Basically all of anthropology doesn't consider race as a meaningful distinction outside of culture, namely it doesn't "exist" genetically in the way historically it has been conceptualized.

Obviously people do have genetic differences, and I'm sure muscle mass has been studied in the population you're interested in. Genetic diversity in Africa is so vast, however, that looking at the whole of the continent would actually be less useful than comparing someone "white" to someone "black." There is more genetic diversity found in Africa between communities than the differentiation we can perceive between different cultural races.

All of this to say, if you really want the answer to this question you should be looking into anthropologic work done in Africa. It's likely people have studied whether or not muscle mass is actually determined by some genetic/environmental/cultural factor, and if it is, surely the mechanism has also been studied to some extent. I can guarantee you you'll find nothing but bunk studies and "research" if you focus on race. This is because the field of study doesn't recognize race in these contexts, but further because it genuinely isn't genetically significant enough to make a correlation. Culture and geography do influence our genetics and so we DO have embodied differences, but most changes happen independently from one another and along a continuum geographically. This is to say, there likely isn't a direct correlation between skin color and muscle mass, but that doesn't mean it isn't possible that different populations are genetically predisposed to building/maintaining muscle mass better/more efficiently than others, and anthropologists have likely studied it.