I don't see why not. There are two or three possibilities here. The most likely is that either the rooster or one of the hens in the breeding pen is not really a black but rather a very very dark blue, which would give you the possibility of blue chicks. Or if the breeder also has blue or splash marans it could be that one of the hens had a secret tryst that no one knew about. Or, since blue is a dilution of the black gene, it is somewhat possible that it could be a random mutation.