The voluntary donation of blood of an unharmed person for the medical benefit of another is not prohibited in any text.
That depends on how one interprets the text. As JW interpret the Bible ( at this time) and it be makes sense, Abstaining from blood includes not flooding your body with gallons of whole blood. The benefits or health concerns is another subject.
blood saves lives
Not arguing that sometimes only a blood transfusion can either prolong life or even stop a person from dying —short of a miracle.
(At one time organ transplants were viewed as cannibalism by JW. Now, some JW have gotten organ transplants.)