One possibility is both dominant and recessive white are present. If the stock is pure for recessive white, it's very easy for the stock to be het. for dom white and you just never know it.

Until those are bred to blacks....

I agree with others the blue probably was present or even deliberately bred into the whites. It's common for white stock to make use of barring, mottle, blue, silver to help clean or crispen up the white.

Maybe wait for the chicks to feather up though.. maybe the initial impressions are not exactly correct. I was surprised by not so quite typically barred or is it blue or is it both? chicks from a show stock white Naked Neck bred to a black. Later breeding showed them to be B^sd, nothing else. Now that was something I never expected to find..