My argument against the blood ban is simple
1. BLOOD WAS CREATED TO FLOW IN THE VEINS. In fact, this is the very reason why blood is considered sacred and equated with life - because of its life-sustaining function as a medium for feeding and oxygenating all body tissue.
2. GOD IMPLICITLY APPROVES OF PEOPLE USING THEIR OWN NATIVE BLOOD IN THEIR VEINS. If he didn't he would require all his worshippers to slit their throats and bleed themselves to death in order to abstain from blood. But he doesn't require them to do this, even though he requires them to abstain from eating blood.
3. GOD DOES NOT APPROVE OF PEOPLE EATING THEIR OWN BLOOD. The scripture condemns eating "any sort of blood". This establishes that the source of the blood is irrelevant. It matters not whether the blood is your own or coming from a donor.
4. LOGICALLY, THEN, THE ISSUE HAS TO BE ABOUT THE USE TO WHICH THE BLOOD IS BEING PUT. Flowing in the veins is ok. Eating it as food is bad. Therefore blood transfusions are legit because they involve putting blood in the veins to do what it was created to do.
Ask a JW what's the difference between using your own native blood in your veins and getting it from a donor in a transfusion. They'll resort to the source argument. You then point out that source is irrelevant because scripture condemns eating any sort of blood and JWs aren't even permitted to transfuse their own blood back to themselves. Then maybe they'll give the reasoning that once it leaves the body it cannot be put back in. You simply respond by asking them why the organization allows the use of heart-lung machine and dialysis which involves the blood leaving the body and reentering. They'll say that it's different because the blood never stops flowing so the equipment functions as a continuation of the circulatory system. Then you ask them well will it be ok if you got transfused with a system that keeps the blood flowing from one person to another without stopping. Then they'll say that they're not here to argue and they'll leave. lol.