there may not be a correlation between race and length, but there is a correlation between race and percentage of spongey tissue.

this is basically what creates the difference between what people call a "grower" vs. a "shower". black people are slightly statistically more likely to be "showers" having a larger flaccid size. I bet this is where the idea stems from. the extreme examples is this will, however, achieve a less hard erection. so it's not all bad for the growers. that said, porn stars have tricks to achieve longer harder erections. Viagra is the absolute norm on professional porn sets. meth and other stimulants also run rampant. it's generally not actually pleasant work.

the racial statistical correlation is weak, and not really worth much. it's about as significant as some peoples being taller on average. in fact, much less so, because "black" isn't a distinct population. i have no idea if there have been better more detailed and representative studies done on this. the only one I've seen just went off of general demographic data sets like "non Hispanic white" or "black". maybe you could argue it's not worth studying, but damnit, everyone kinda wants to know. it's all talked about often enough. we should really get to the deepest bottom of it just to end the argument. even though proving things with data never ends arguments.