It should be obvious that a blood transfusion causes no harm, bodily injury or death to the blood donor, which is not the same as when the Bible talks about pouring the blood out on the ground or shedding blood, which= death.
What rule do you live by?- A: all things are forbidden, unless they are explicitly allowed or,
B: all things are allowed, unless the are explicitly forbidden?
The Bible does not mention blood transfusions at all, eating blood yes, but not transfusions.
God, with his all seeing eye could easily see in to the future and know that blood transfusions would soon be invented, so he could have, by his spirit, made the Bible writers use more explicit language that would have covered the use of blood in transfusions. Could have , but didn't.
However, 99.9% of the world sees no religious problem with blood transfusions- it's only JWs who are stubbornly locked in on a Bible interpretation that denies a life saving blood transfusion for it's members. No doubt, it would be hard for JWs to switch sides after so many JWs have died over the last 50+ years. - if they did reverse themselves, then the issue of innocent lives lost and blood guilt would have to be addressed. Just how do you think Jehovah would punish just such a religion as that? What would be a just punishment for manslaughter done in the name of God?