I guess I didn't think of it like that. Well, ok. But since magic is the grand confuser of stuff, can magic account for the in-flight change of blood type?
Our half elven spellbinder want to use it for healing purposes but can't because the other player characters are all of some other kin.
Hmmm, I'd argue that half-elves can take
the blood of elves, humans and other half-elves. They have the blood of humans and elves running through them, after all.
If they want to give
their blood to an elf or human, I'd say they should roll a number of dice equal to the number of attribute points being given, and if any 1s turn up, they make the recipient sick.
Give the recipient a critical injury "blood poisoning" and have it provide a -1 to every
skill roll. Or something similar.