Picture of Mongolian Spots
Mongolian spots are very common in any part of the body of dark-skinned babies. They are flat, gray-blue in color (almost looking like a bruise), and can be small or large. They are caused by some pigment that didn't make it to the top layer when baby's skin was being formed. They are harmless and usually fade away by school age.
Text: "Your Newborn's Skin and Rashes", WebMD Medical Reference, "Skin Conditions: Pigmented Birthmarks", WebMD Medical Reference