It's called the Urogenital raphe (pronounced rafe). It is a scar, sort of.
That lower pelvic area is open during the early phase of embryonic developement. If you are female, it remains open, for the most part (think vulva/female genitalia). For us guys, it gradually closes, starting in the anal area, progressing anteriorly sealing the scrotum (by the way, your nuts are separated by a septum of tissue inside your scrotum, which develops from the closing raphe,) along the base of your penis toward the head. There are rare developmental abnormalities in which the raphe fails to close completely, resulting in the meatus of the penis (your piss hole) appearing anywhere along the lower portion of the penis. The worst case is a hypospadias opening in the scrotum, but the more common site is somewhere below the glans. Mild hypospadias is fairly easily corrected, but it is really important to take a good look at the baby's penis before he is circumcised. That foreskin is needed for the plastic surgical repair.
As to why some guys are darker than others, guess it's just a matter of how dark their other skin is. Genital skin tends to be much darker than our regular skin anyway. I've seen guys with milk white skin on their butts, but their scrotum and penis are almost black. Probably some evolutionary benefit there, although I haven't a clue as to what it might be.
Hope I've answered your question!
Dr Danny
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