Most Asian babies are born with a blue-grey patch on the lower back or buttocks. It’s called the Mongolian spot, and it’s harmless.
What happens: while the fetus develops, some melanocytes (the pigment-making cells) get stuck in the dermis instead of reaching the surface. Melanin seen from that deep looks blue, because the skin above scatters the light (the Tyndall effect). The spot usually peaks around age 2 and fades by 6 to 10, although sometimes it stays for life.
How common it is depends a lot on ancestry, and also a lot on which study you read. Around 80% of Asian newborns is the usual figure; some sources give over 90% for Black babies, similar numbers for Native American and Polynesian ones, and 10% or less for European ancestry. A prospective US study found much lower rates, so take every percentage as approximate.
The name has no anthropological basis (it shows up everywhere), and doctors now prefer congenital dermal melanocytosis. It matters: mistaken for bruises, it has triggered false suspicions of child abuse.
It also has a place in literature: part two of La vegetariana by Han Kang is titled “The Mongolian Mark”. A video artist becomes obsessed with the fact that his sister-in-law Yeong-hye still has hers as an adult, and that one image ends up wrecking his marriage and his life.