I agree, they do have excellent genetics and most of them are gifted mesomorphs.
Gotta say that Italians run a close second. I happen to be Italian and started working out at 18 years old. I weighed 170 at 5'11". In 2 1/2 years I reached 210lbs. with 18" arms COLD with a 48" chest just standing in a relaxed postion (51" flexed). And this was attained all natural, I still am natural at 33 years old and weigh 217 with a much better and bigger pair of legs. Getting ready to do my 1st cycle!!
A lot of Italian guys I know have same genetics and develop pretty fast and are very strong.
But I gotta hand it to you, Black men are superior in this aspect.