Breed the blue to the blue, and dont let any other eggs in the nest, then you can see it happen right before your very eyes.
It is simple.
The gene responsible is called a Co-Dominant gene. That simply means, if a bird has one copy, then you can see it (the bird will be what we call "blue") But two copies makes the gene work twice as strong. So we see Blue expressed twice as much. Those birds are called "splash."
So if a bird is BLACK, then it has zero genes to make it blue, hence its blackness.
If it black, it cannot pass a gene to its offspring to make it blue.
If a bird is BLUE, then when the cell divides, and the genes unravel, it will pass one of those genes to its offspring, and either that gene will tell the bird to look BLACK or BLUE
A SPLASH bird has two genes for BLUE, so either one that is gives the offspring will tell it BLUE
Splash X Splash = Splash
Splash X Black = Blue
Black X Black = Black
Blue x Blue = Blue Black and Splash