Regardless of ethnic background, it is normal for your penis--especially when erect--to be darker than the rest of your body.
A lot of features of the human penis are meant purely as visual displays, left over from evolutionary selection pressures based on promiscuity and sexual competition for females. They served to attract the attention of females and scare or subdue rival males. That's mainstream biology talking: just googlesearch "penis display function" or "penis sexual selection," or check out a book called "The Third Chimpanzee" by Desmond Morris.
Humans have the biggest penises, both absolutely and in proportion to body size, of any primate. We're the only primates with such "chunky" equipment--our closest relatives, the chimps, are hung like pencils. Even an averagely-hung man has quite a bit more than he needs if the sole function were insemination. We can throw erections bigger than we biologically need, equipped with all that pulsating movement and a color distinct from the nearby torso, because back in the day it was a great way to advertise how healthy and well-nourished you were.