Ok Sassy, I'll give you an outline of the proof. Later I'll give you a link to a full article.
As a side point, in In Search of Christian Freedom Ray Franz makes some excellent points about the Society's misuse of various scriptures to enforce its blood ban.
The Society bases its ban on transfusions essentially on three groups of scriptures, Acts 21, Leviticus 17 and Genesis 9:4. They claim that Acts 21:25 means that all Christians must not eat blood under any circumstances. Then they make the scripturally unsupported claim that a transfusion is the same as eating blood, concluding then that the Bible prohibits transfusions for Christians. They claim that Leviticus 17:10-14 and related OT passages give Christians a principle which reinforces the "blood ban" in Acts 21:25, namely, that the Jews were prohibited, on pain of death, from eating blood -- even the blood of animals that died of themselves or were killed by predators (the point being that such carcasses had not been properly bled). They finally claim that these prohibitions are based on Genesis 9:4, which states (NASB):
"Only you shall not eat flesh with its life, that is, its blood."
They claim that this statement amounts to a law for all humans to avoid eating blood, and that it's the basis for the explicit prohibition to eat blood by Jews and Christians. You'll see that this claim is specious.
As usual, the Society misinterprets many things in this sequence of claims and ignores crucial points and scriptures.
First, it's clear in context that Acts 21 is talking about what things gentile Christians ought to avoid so as to not offend their Jewish Christian fellows. This centered around the question of circumcision. The Jews had many cultural biases due to the Mosaic Law, some of which were so ingrained that they thought that even gentiles ought to observe them. Circumcision was again the prime example. The account describes how a group of elders in Jerusalem decided on a compromise -- the gentile Christians should observe some of the Jewish cultural bans: on eating blood, eating meat sacrificed to idols, eating things strangled, and fornication as defined by the Jews (fornication as defined by non-Jews could be rather different).Galatians 2:14 states: "But when I saw that they were not straightforward about the truth of the gospel, I said to Cephas in the presence of all, "If you, being a Jew, live like the Gentiles and not like the Jews, how is it that you compel the Gentiles to live like Jews?"" What is not clear from the account is whether this was meant to be just a temporary measure designed to avoid offending Jews until they got over their cultural biases, or was a permanent thing intended to be observed for all time by all Christians. This uncertainty is proved by 1 Corinthians 8 and 10, which specifically states that there is nothing inherenly wrong with a Christian eating meat sacrificed to idols, but that it's wrong only if it offends someone with a weak conscience who would view eating such meat as an act of worship. In view of this uncertainty, one cannot dogmatically claim that Acts 21 provides a basis for an absolute ban on eating blood. In reality the Society's only justification for its absolutism here is the claim to spiritual authority of those who claim to comprise "the faithful and discreet slave."
Second, it's obvious that eating blood is completely different from being transfused. The Society's standard argument that transfusing is just bypassing the usual digestive process is easily defeated. When blood is eaten, it's broken down into its component amino acids and other chemicals. But when it's transfused, it just continues to act like blood. So a transfusion is really an organ transplant, and the blood is no more eaten than a kidney is eaten when it's transplanted. If a doctor told you to avoid eating meat because it was bad for your own health, and later he told you that you needed a kidney transplant, he'd think you were nuts if you complained that he was being inconsistent.
Third, and most important for my line of argument here, is the fact that the Bible specifically told the Jews that, while they themselves couldn't eat the blood of an animal that died of itself, they could give such a carcasse to gentiles. Therefore, if the God that the JWs believe wrote the Bible is self-consistent and doesn't give explicit permission for a group of people to violate his sacred laws, the statement in Genesis 9:4 cannot be a law against eating blood by all people for all time.
God gives permission for gentiles to eat blood the blood of dead carcasses in Deuteronomy 14:21:
"You shall not eat anything which dies of itself. You may give it to the alien who is in your town, so that he may eat it, or you may sell it to a foreigner, for you are a holy people to the LORD your God."
This, along with the context of Levitcus 10, shows what the intent of the Mosaic Law was regarding blood: it was to be used by the Jews only in the ceremony of atonement and in blood sacrifices. This was consistent with other capital offenses with respect to eating certain foods, such as fat. While the Law prohibited eating fat, on pain of death, it certainly doesn't apply to Christians.
This, by the way, is yet another area in which JW leaders pick and choose the biblical passages they want to observe. They rightly teach that the Mosaic Law doesn't apply to Christians, but turn right around and use it (claiming that Christians must obey what they call "principles of the Law") as a law for Jehovah's Witnesses!
Since Genesis 9:4 doesn't constitute a ban on eating blood per se, what does it mean? A careful consideration of everything the Bible says about blood leads to a simple conclusion: it means that when a person kills an animal for food he must give a token of respect to the Creator of life by pouring out the blood and not eating it. This is the only conclusion consistent with Deuteronomy 14:21 and with the obvious fact that nowhere does God give any laws in the Bible that require people to pour out the blood of animals when they kill them for purposes other than eating them (excepting of course the specific ceremonial requirements for Jews). So Genesis 9:4 is not essentially about blood, but about showing respect for life and its creator.
This theme of showing respect for life is shown by a number of Bible passages, where people broke a specific prohibition in order to save life, and retained God's approval.
As for whether it can be argued that it's good for Christians to give blood when it can save a life, Jesus stated in John 15:13: "Greater love has no one than this, that one lay down his life for his friends." It seems to me that if a person can properly sacrifice himself for another (which could be claimed to be suicide, which the Bible seems to prohibit), then sacrificing a bit of blood to save a life would be a fine thing -- especially since it's now clear that there are no biblical prohibitions against transfusions. And even if Acts 21 really does prohibit blood transfusions, Jesus argued strongly that it's ok to break certain laws in order to save life -- even the life of an animal (cf. Matthew 12:12). The point here is precisely what Jesus condemned the Pharisees for not understanding: "I desire compassion rather than sacrifice."