To me the Punnett square method is the best explanation.
If you assign the Black = (BB), Blue =(Bb), one blue gene creates blue, and Splash as (bb), two blue genes. Each of the parents contribute one color gene. The result is a mathematical proportion, theoretically at least in large numbers.
IE,
Start with a black (BB) and Splash (bb), each offspring will get one B and one b,
therefore all chicks will be (Bb) = 100% Blue.
With Blue (Bb) and Blue (Bb) matings
Half of the chicks will get (Bb), 25% get (BB) and 25% (bb)
The last two are the ones in my mind that don't seem to make sense proportionally, but a bit more logical if you think about each gene in a punnett square matrix....
Splash (bb) and Blue (Bb)
50% of the offspring will get (bb) and 50% (Bb)
Black (BB) and Blue (Bb)
50% get (BB) and 50% (Bb)