Mrs.V-Perez -- To expand on what Phil said.....
A split to blue is significant for Gouldians because you never want to pair a blue and a blue Gouldian for breeding. This is due to the relative newness of this mutation, which results in a fairly small genetic pool, which results in a high chance of genetic flaws in the offspring.
So in order to breed a blue Gouldian, you want to pair two Goulds who are split to blue (they're carrying the blue gene, but don't show it) or you could do a blue with a Gouldian who's split to blue.
The only way to know if a bird is split to a particular trait is through breeding and looking at the offspring and basically reverse engineering their genetics OR by knowing the bird's parentage (something only a breeder would know.)
To offer an example of how you'd determine splits through breeding....
I have a red headed male and a black headed female.
You need 2 black-head genes to get a visually black-headed bird.
So if my red-headed male is split to black (in other words, he had a black-headed parent and a red-headed parent), then I can get black-headed birds since they can get the 2 required genes to have a black head --- one black gene from dad and one black gene from mom. I can also get red-headed birds from this pairing. (50% red and 50% black)
If my red-headed male had two red-headed parents, then he has no black-head genes. Therefore, there's no way this couple's offspring could get the 2 black-head genes required to display that black-head trait. They could only get 1 black-headed gene from mom.
So all male offspring would be red split to black.
And all females would be red-headed. (No splits, since they're heterozygous - ZW and only have one Z, and the Z is where the head color gene is found - whereas the male is homozygous, ZZ, so he gets 2 head color gene. One is visible and one is hidden.)
In sum, if I get a bird with a black head, I know for certain that my red-headed male had 1 red-headed parent and 1-black headed parent.
If I get all red-headed birds, it's likely the male had 2 red-headed parents. (Though not certain as I only have 3 chicks. Small sample size, and in theory, I could get all 1 head color as the luck of the draw. Just as I could get all males or all females. )
If that's all chinese to you and you're a more visual person, there's a couple good graphics that illustrate how splits work in the above-mentioned pairing here:
http://wp.me/p4JWrl-eu