Andalusian chickens show incomplete dominance for feather color. a cross between a white bird and a black bird produces offspring that have blue feathers

Even with this result that, the cross between white and black birds produces offspring that have blue feather doesn't make a blue feathers chicken a purebred, but because they have different alleles. In some cases a cross breed between F1 blue chicken gives blue chicken majorly and few white or black chickens.