Acts 15:29 plainly says to abstain from blood.

It does, and the Witnesses still apply it to eating blood and to whole blood transfusion. But their own article now says the Bible does not comment on products derived from blood, which concedes that the step from “don’t eat blood” to “don’t accept a transfusion” is an interpretation rather than the text. It is hard to hold that a whole blood transfusion is eating blood while a transfusion of red cells, the largest component of it, is not. Acts 15 addresses fellowship between Jewish and Gentile believers alongside food sacrificed to idols, and most Christian traditions have read it that way.