Those are black with white barring. There is no blue involved.
The females have one copy of the barring gene, the males have two copies of the barring gene, which is why they are lighter.
Barring is on the Z sex chromosome. Females have sex chromosomes ZW (one Z that can have barring, one W that makes them female.) Males have sex chromosomes ZZ (so they can have no barring, one copy of the barring gene, or two copies of the barring gene.)
A chicken with two copies of the barring gene is lighter colored than a chicken with one, and is always male. This can sometimes be used for sexing chicks in certain breeds (the Cochins you have now, Barred Rocks, Cream Legbars, Bielefelders, anything called "autosexing.") The reason it can only be used "sometimes" is because the light/dark difference is not always clear enough to see easily.
A chicken with one copy of the barring gene, or no copy, could be male or female. But if you know the parentage, you sometimes know the gender as well:
A rooster with two copies of the barring gene will give one copy to each chick. If the mother has no barring, that means the chicks will each have one copy of barring, and will look alike. If the mother has barring, the sons will have two copies of the barring gene, the daughters will have one copy, and the chicks are sexable like your current Cochins.
A rooster with one copy of the barring gene will give it to half his chicks, and the other half will get not-barred. This means half the daughters will be barred and half not-barred. If the mother has no barring, half the sons will be barred and half not-barred. (These chicks are not color-sexable.)
For a rooster with one copy of the barring gene, who gives it to half his chicks, if the mother has barring, half of the sons will be double barred (two copies of the gene) and half will be single barred (one copy of the gene.) Half the daughters are barred and half are not-barred. The double-barred sons can be recognized by color and so can their unbarred sisters, but the single-barred chicks will include both males and females.
A hen with the barring gene will give it to her sons but not her daughters (because she is giving her daughters the W chromosome to make them female.) If the father is not barred, that makes barred sons and not-barred daughters, which can be sexed by looking for the barring on the males (Black Sexlinks are made this way.)
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