Andalusian chickens show incomplete dominance for feather color. a cros between a white bird and a black bird produces mostly blue feathers. a cross between two f1 blue chickens produces mostly blue chickens. are the blue chickens purebred?

No, They are not purebred

Explanation:

Incomplete dominance mean that the two alleles does partial dominance over each other leading to a brand new trait being expressed ( blue feather). In addition, there was no allele representing or coding for blue feather  from the beginning of the generation; blue feather was only the result of the interaction of the partially dominant alleles of white feather and black feather.