One possible explanation for the occurrence of darker skin is a condition known as post inflammatory hyperpigmentation (PIH), which is simply an over-production of melanin in the affected area. This condition typically follows an injury to the skin; irritation from aggressive masturbation or sex is a likely cause for PIH of the penis. A penis affected by PIH may have areas of skin that appear darker brown, or bluish or gray. While this may cause some social embarrassment, it is not contagious and generally fades over time.