I am sure pure Black pigeons will produce only pure Black pigeons. In your case, your pigeons are not pure Black. They are genetically split for Red and the Black is masking this feature. If you are sure that no other pigeon is involved, then this Red YB is recessive to the Black and may be a pure Red, but could also be split and carrying a Red gene. I am not an expert in pigeon genetics, but suspect this is a case of incomplete dominance.