This was in a thread and was posted by Speckledhen that she linked from Bev Davis' website. Hope it helps.
Blue color is the result of a third gene. It is a modifying gene for black pigment, and is located at a parking spot called "Bl." The choices (alleles) that can go into this parking spot are basically black pigment (bl+ = black pigment allele) or a bleaching allele (Bl = blue allele, which really is the black pigment allele with a modifier that bleaches the black pigment). If you think of the blue allele (Bl) as a bleaching version, then you can visualize how if a bird gets one dose of blue allele (Bl,bl+), it turns out blue in color, and if a bird gets two doses of blue allele (Bl,Bl), it gets double-bleached all the way to splash. And the blue allele acts wherever there is black in the bird, so wherever the bird WOULD HAVE BEEN BLACK, it is now bleached either to blue, or all the way to splash, depending if it gets (one blue allele and one black pigment allele), or (two blue alleles).
Basically splash birds have two copies of the blue gene, that is what I understand anyway!
Blue x Blue= 50% Blue, 25% Black , 25% Splash
Blue x Black= 50% Blue, 50% Black
Blue x Splash= 50% Blue, 50% Splash
Black x Splash=100% Blue
Splash x Splash= 100% Splash
Black x Black= 100% Black