A Japanese friend tells me the blue is in reference to the 'Mongolian blue spot'[0] that most Asian, Pacific, Native and African American babies tend to have...

A Japanese friend tells me the blue is in reference to the 'Mongolian blue spot'[0] that most Asian, Pacific, Native and African American babies tend to have.... Basically bodies with lots of melanin in them; it's a cluster of melanin at the lower back, sometimes mistaken for a bruise by carers unfamiliar with the phenomenon. [0] - https://en.wikipedia.org/wiki/Mongolian_spot https://en.wikipedia.org/wiki/Mongolian_spot EDIT: Oh, it's mentioned in the wiki article: > The Mongolian spot is referred to in the Japanese idiom shiri ga aoi (尻が青い), meaning "to have a blue butt", which is a reference to immaturity or inexperience.