My guess is that it is a splash. The blue feather gene is partially dominant and it modifies what would normally be a black feather. If you get two "blue" genes at that gene pair, you get a splash (mostly white with black markings). If one is a "blue" gene you get a blue chicken. If none are "blue" you get black.
If you cross a blue with a blue you can get Splash, Blue, or Black.