I'm not into men, so I'm not basing this off of personal anecdotal "experience with them.
It's possible that black men may be more likely to be "showers" instead of being "growers", if you think about the environment their ancestors are from this actually makes sense. It's well understood that humans physical proportions have adapted to the type of climate their ancestors are from, longer thinner bodies for hotter climates, shorter stockier ones for colder climates, etc. I think this could possibly be the best explanation for this, when you are cold your dick shrivels up, when you are hot it hangs.
Also, not sure if this has been mentioned but I'm aware of the fact some black men feel the need to live up to the BBC myth. Those who are indeed big may be more likely to sleep around and flaunt it compared to more average black guys, as they are more confident due to living up to that reputation.