Blue X Blue does not give 100% Blues
Actually this is the way their matings go:
Blue X Blue = 50% Blue, 25% Black , 25% Splash
Blue X Splash = 50% Blue , 50% Splash
Blue X Black = 50% Blue, 50% Black
Black X Black = 100% Black
Splash X Black = 100% Blue
Splash X Splash = 100% Splash
Only way to hypothetically get 100% Blue, is to do Black X Splash.
Self Blue does breed true though.