I mean it is pretty normal among both sexes for the genital area to be darker than most of the skin. It's just that some artists take that to rather extreme levels.
Melanin tends to concentrate more in certain parts of human body than in others, the elbows and genital zone are the most common ones. As a side fact most people who work in the porn industry "bleach" their genital areas just to make them look more pleasant to the eye
