Mongolian Blue Spot

Mongolian blue spots are flat bluish- to bluish-gray skin markings commonly appearing at birth or shortly thereafter. They appear commonly at the base of the spine, on the buttocks and back and also can appear on the shoulders. Mongolian spots are benign and are not associated with any conditions or illnesses.

http://www.nlm.nih.gov/medlineplus/ency/imagepages/17262.htm