The main reason why a blue cat gives birth to a black cat is because of impure blood or genetic abnormalities in the ancestors. Under normal circumstances, if a black cat and a pure blue cat mate, it is possible to produce pure black, pure blue or blue-black cats. But if both are blue cats, it is likely that at least one of them has a bloodline problem, or is caused by genetic abnormalities in the ancestors, because purebred cats are essentially unified through multiple breedings. Stable genes have been produced, but occasionally there will be one or two exceptions. However, according to international standards, generally such special colors are disqualified.