If you look at the context of Acts 15, the commands given, are given to Gentiles BECAUSE Jews with weak consciences would be stumbled if they saw Gentiles doing these things.
These were not eternal, absolute moral commands from the Almighty. They were temporary, cultural, ethnic instructions to help the Jews and Gentiles get along together when they were brought into the same Congregation.
For example, one of the commands given is that Gentiles must not eat any meat sacrificed to idols. Yet, the Apostle Paul wrote many times telling Christians that it is "up to your conscience" whether to eat such meat or not. Therefore, we can clearly see that the commands in Acts 15 were not designed to be eternal moral commandments.
In addition, the command against eating blood is referring to animals that have been killed by humans-- the same prohibition that was found in the Old Testament. The Law of Moses made a clear distinction between eating blood from animals that were killed by humans and animals that died of other causes.
The purpose of this is to show that if humans take a life, even the life of an animal, they must honor and show thankfulness to God by demonstrating that this animal's blood (life) actually belongs to God.
There was absolutely no severe punishment in the Bible if a Jew ate the blood of an animal that was NOT killed by a human.
One more point to consider: Jesus made it clear that God prefers mercy over sacrifice, and that King David eating the Holy Showbread to save his life was more important than following the technical written laws of the Old Testament. So why should blood be any different? If someone can save their life by taking in medical blood, shouldn't that be more important than following the technical written rules?
Finally, in blood transfusions, no one's life is taken, instead people's lives can be saved. If no life is taken, then the Bible commands about showing respect and demonstrating that God owned the life you just took, would not even apply to this situation.
Bottom line: Using medical blood should be up to each person's conscience, not a mandatory Law.