Here is the deal. I do not know what happens if you get blood from a vaccinated person and you are unvaccinated. I would find it highly unlikely that there is not remnants of spike protein in that blood. But I honestly do not know.
Blood in our body is not “permanent”, it “turns over” every 30 days or so, depending on the type of blood cell it is. Some live longer than others do. The reason I would hypothesize that there could be risk is that we KNOW the spike proteins from the injections do migrate to the bone marrow. What does the bone marrow do? It creates blood cells. Red blood cells. Which, that is what you get in a typical blood transfusion. Packed red blood cells.
So how long does the MRNA hang around the marrow? Does it permanently alter it forever? If you create abnormal red blood cells, are those safe for anyone to receive as a blood transfusion recipient? One has to wonder given the rapid increase in cancers, such as blood cancers, since the mRNA injection was introduced. Ethical Skeptic has covered this extensively.
If you have an elective surgery that may require a transfusion, you can donate your own blood and bank it prior to surgery. Get your blood type cross and matched and see if you have a friend/loved one with your blood type who is not vaccinated and ask them to donate blood prior to your surgery.