If blue birds (genetic code Bb) are mated together, they throw a percentage of ‘off' colours in their progeny (youngsters), the probability being 25% blacks (BB), 25% blue splashed whites (bb) as well as 50% blues (Bb). You need to breed a sufficiently large number to achieve this percentage - since this is a probabilty.
Black and blue splashed whites are homozygous and the blues are heterozygous - i.e. they produce equal number of black and blue splashed white gametes. This can be tested by breeding the blacks or the blue splashed whites with themselves and each will breed true to its respective type. This means they will breed true. The blues will not breed true, they carry two colour genes and no amount of breeding selection will ever make them breed true.
Blacks and the blue splashed whites can be useful. If they are bred together, their progeny will be 100% blue therefore producing twice as many as when just mating blues together. There are different shades of blue and some come out darker than others so if you are wanting the birds for showing or breeding, you only get about 50% of the 'blues' that show the desired shade of colouring.
Mendel's law of inheritance, says each chick receives two genes, one from the mother and one from the father. Since blue is made up of 2 colour genes - black (BB) and splash (bb), the laws of probability say we have 4 different possibilities when crossing Blue with Blue. BB, Bb, bB, bb.
The following combinations are achievable: