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.