It is not scriptural. Yes, the Bible says to not eat blood. Never mind the fact that transfusing blood is not the same thing as eating blood but rather is an organ transplant (which the Society otherwise allows, even tho some organs do contain some blood). What the Bible does not say is that one must not eat blood under pain of death. In fact, Jesus several times advised that the usual commandments can be set aside if life is at stake. He mentioned the incident of "what David did in his time of need when he and his followers were hungry, how they went into the House of God when Abiathar was high priest, and ate the loaves of offering which only the priests were allowed to eat, and how he also gave some to the men with him" (Mark 2:25-26). In unusual circumstances, in one's time of need, the usual demands of the commandments can be relaxed. The same thing was the case with resting on the sabbath, Jesus says that saving life is more important than observing this law: "Is it against the law on the sabbath day to do good, or to do evil, to save life, or to kill?" (Mark 3:4). This was the same idea that many other Jewish rabbis had, that almost any law could be set aside in order to save life (as this would fulfill the bigger law in Leviticus 19:16-18, endorsed by Jesus, that one must "love your neighbor as yourself" and must "not jeopardize your neighbor's life"). This is very basic jurisprudence. The problem with the Society is that it has no sense of jurisprudence and demands that some laws must be obeyed to the point of death, even though eating blood was never a life or death matter in the Bible -- unless it were to hold off dire starvation, and as Jesus indicated in Mark 2:26-26, one can do what needs to be done to be fed in one's time of need.
The reason why animal blood is not eaten, btw, is because meat requires the killing of the animal and the blood is offered to God as an atoning sacrifice (Leviticus 17:10-14, Deuteronomy 12:23-28), as the life of the animal was believed to be in the blood. Blood transfusions are entirely different because NO KILLING OCCURS when blood is donated. People are not slaughtered in order to get the blood used in transfusions. Since life is not taken, there is no life to atone for (Leviticus 17:11). The atonement ritual described in Leviticus, which involves pouring the blood to the ground and covering it with earth, simply has no relevance. I do wonder tho what the Society does to atone for all the deaths of JWs who fail to accept transfusions in their "time of need" in order to "save life". Does the GB not jeopardize their neighbor's life by forbidding a medical procedure on the basis of blood that is not eaten and does not need to be atoned for?