Searched around and found different studies, one of them said black didn't have increased amount of testosterone when compared against white people but did have increased amount of estradiol, a female sex hormone so i guess this one couldn't help them since female sex hormones have the opposite effect of increased muscle mass, but also found one that said black people have incresed amount of testosterone(free test and not free test and also DHT) and for that reason they are more predisposed to prostate cancer, this is widely seen in america it seems where black people have the higher rate of prostate cancer when compared to white males.
After analyzing the studies i think more studies need to be done before i can say for certain anything, we got studies saying one thing and other saying another thing, so regarding the first question i think more studies need to be done, which is something extremely difficult to do nowadays because any studies that aim to compare white males with black males are labeled racist by the common person, when from a scientific point of view i find it intriguing. If a gene is discovered we could insert it into animals to increase the amount of muscle mass they have so that it increases meat production for example, or we could upregulate it to increase the amount of muscle in cancer patient, HIV patient, etc...