Japan blue is easly identified by blue coloration of peduncle. Probably comes from feral population of Poecilia reticulata living in Japan waters. It is y-linked. Some breeders belive it is possible to jump it on X chromosome – others belive it is not possible.
It is worth mentioning that population very similar in look to Japan Blue was found in South America. So called Lac du Rorota guppies. Never kept them, but hope so in future.