A large, blue-gray birthmark.

A mongolian spot is a large, blue-gray birthmark. It can be mistaken for a bruise. It's most common in Black children and those of Asian descent. It's also called a dermal melanocytosis and a slate gray nevus.

Mongolian spots usually appear on the lower back and, less commonly, on the buttocks or shoulders.

The birthmark tends to fade during early childhood and needs no treatment.