If you want to get technical, it's called congenital dermal melanocytosis in Western medicine, and it often results in a distinctive blue birthmark on the bottoms of babies from East Asia. If you want to get nearly poetic, you'll want to read Jennifer Hope Choi's piece at Longreads headlined "My Mongolian Spot." In the opening lines, she writes, "First you should know: I was born with a blue butt. So was my mother." As Choi explains, the birthmark typically disappears in four or five years, and science has a perfectly boring explanation for it involving melanin during embryonic development. Better is the Korean folklore tradition involving the "Grandmother Spirit," who causes the spot by slapping newborns to life.
Choi provides a sense of what they look like: "a wavy-bordered blue spot not unlike that of Rorschach's inkblots, blooming across our tiny bums—blue like ice-cold lips, blue like the ocean at midnight, Picasso's most melancholic bluest of blues." And she uses the birthmark as a launching point to explore her Mongolian ancestry, her identity as the daughter of South Korean parents who was born and raised in Los Angeles, the racist undertones of the term "Mongoloid," and much more. And if she has a child someday, she will tell her all about the Grandmother Spirit. "I want her to know that being born blue is exceptional, some magic in our chemistry, a bright and bruised and blooming sign that says, if only to each other, we belong." Click for the full piece.