Congenital melanocytosis, previously known as Mongolian spots, is a very common condition in any part of the body of dark-skinned babies. The spots 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.
In most cases, no treatment is needed for the birthmarks themselves. When birthmarks do require treatment, however, that treatment varies based on the kind of birthmark and its related conditions. Read more about newborn's skin and rashes.
Slideshow: Slideshow: A Visual Guide to Birthmarks