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 tho.
Grzegorz Prusinowski examples:
Those males were created in unintentionally cross to females that might have been M. parae hybrids. They are big and the japan blue gene expression is not so standard.
Bolesław Gaweł Examples: