one possible reason that i have heard before has to do with slavery.
when black males were brought over from africa by slave traders the larger, stronger males were obviously more popular because of their ability to do more work. these males were also "put out to stud" supposedly and this would mean that on average the genes for larger stronger black men would be carried on.
what does this have to do with penis size? well, from what ive seen there are lots of studies that say race isnt a factor in dick size and that on average no one race has the advantage. but theres also studies out there that say race is a factor and that black guys are bigger and asians tend to be smaller.
the fact is that surveys and statistics can be made to prove anything u want and generally are not as reliable as people seem to think they are.
the only real indication of penis size is overall physical size. the human body tends to be somewhat proportional. meaning that if a guy is 6 foot 7 it doesnt matter what color his skin is hes gonna have a bigger dick then a guy whos 5 foot nothin. and while i wouldnt say that my personal experience is definitive proof i would say that i have seen more large(in height) black men then that of any other race.
whether this is a direct result from some sort of selective breeding of the black race during salvery seems to me to be one of the most logical answers.
simply because they are black just isnt enough, in my mind atleast because the color of a persons skin is linked to a hormone that really has nothin to do with anything else in the body, let alone dick size.
thats my 2 cents worth