Horse have 7 different blood types with extra variations, but since I like the idea fluffy DNA is a blend on dozens of species I sort of assume they are universal donors since they’d have coding in place not to react to all the different DNA.
One of the common head canons is that fluffy genes are screwed up enough that they cant recognize non self material allowing for easy transplant, transfusion, or horrible Dr. Moreau experiments.
Another comic that gets me wondering what a fluffy’s rejection rate is. For my own stories I think I’ll headcanon that a fluffy’s body never actually rejects implants or transfusions. But if you replace a fluffy’s limb with one of a different color the fluffy itself might reject the limb and begin attacking itself.