Slate gray nevus

A slate gray nevus is a usually harmless, large flat patch. It may show up on the lower back, buttocks or shoulders. And the color ranges from deep brown to slate gray or blue-black. It can be mistaken for a bruise. This type of birthmark is more common in Black, Native-American, Hispanic and Asian babies than in white babies. A slate gray nevus tends to fade during early childhood and needs no treatment.

It's also called a congenital dermal melanocytosis and was formerly called a Mongolian spot.