Board Thread:General Discussion/@comment-968312-20150209182531/@comment-25900245-20150210204525

"[...] all of the Ark residents are the same blood type, Type O (the universal donor blood type). This is fairly easy for them to engineer genetically [...]"

http://the100writers.tumblr.com/post/104088766213/serious-question-here-how-are-none-of-the

So, yeah, the sky people should be able to donate blood to anyone else, but grounder to grounder transfusions would require some further testing.