There is nothing sacred about blood. It is merely a heterogeneous mixture of various compounds and cells that is necessary to nourish and sustain complex animal life. It is logically no more 'sacred' than urine. Actual Life is the only thing that can, for good reason, be described as 'sacred'. Blood has become such because of it's symbolic and superficial connection to life and death. Urine is equally important to life, it merely serves a different function. If the body had no way to expel toxins, they would eventually build up and the body would essentilly die of poisoning. If blood is venerated as sacred because of its necessity to life, then so should we venerate urine. Yet, urine gets deemed 'icky', and blood 'sacred'.
Anyway, blood, at some point in man's history became symbolic of life. Probably because when an animal or human receives a fatal wound, blood pours out of it, and death ensues. Therefore, the loss of massive abounts of blood = death, and by logical extension: blood = life. Hold onto your blood, hold onto your life.
Enough of my rant and more to the point... Blood transfusions are ironically temporary. Eventually those cells and compounds in the transfusion are replaced by those that have been produced by your own body. So given a certain period of time, your blood is once again "pure" as the OP put it.