There have been large studies on testosterone levels and race. Black and White male groups had no significant statistical difference in testosterone levels. This is reported in the literature already.
I believe some black males have a higher % of Type II muscle fibers which are the ones to hypertrophy the most...particularly in their arms and shoulders. Of course, among any race there will be variations in genetics. We're all seen some black guys grow very quickly, particularly upper body, and some that have poor responce to resistance training as well.