The blood type problem between a mother cat and a kitten was actually quite critical. Not only was it dangerous for non-blood related mothers to pair with kittens, but even if the blood type between the mother cat and the kitten was different, it would be dangerous for them to breastfeed.
It might sound incredible, but if the female cat was Type B and the male cat was type A, and the kitten with type A blood was born, the kitten could not drink the milk of the female cat. Instead, the owner should replace it with goat's milk. Otherwise, the kitten might have a serious hemolysis reaction, ears and tail necrosis, and it might even die without any warning.
On the other hand, the female cat was type A, and the male cat was Type B. It was not a big problem to give birth to a kitten with Type B blood, because the antibodies in type A blood mother milk were less, so it would not have a significant impact on the kitten.