This particular topic is not interesting to me. Don't know why... just doesn't top the list of issues I have with the WT organization. That said, the ONLY reason JWs do not permit blood transfusions is because Acts 15:29 says to "abstain...from blood." That's it.
As someone on this thread astutely pointed out, (way back on page 3 or 4 or 5 or 6... can't remember), Acts 15:29 also says to abstain "from things sacrificed to idols," and this was downplayed by Paul at 1 Cor 8:7-13, making it a conscience matter, but to make sure it does not stumble your brother. Additionally, Hebrews 8:13 shows that the old covenant was obsolete and "is near to vanishing away." From this, we can conclude that the laws in the Mosaic Law Covenant were mostly in effect when Acts 15:29 was penned, but were on their way out, and by the time Hebrews was written, were mostly gone.
Also, Hebrews points out that because there is a new priest "of the order of Melchizedek," that is, Christ, there is a new law. "For since the priesthood is being changed, it becomes necessary to change the Law as well." (Heb 7:12)
Sadly, JWs want to be part of the old "order of Aaron" (Heb 7:11) and thus want to hang onto the old Law, even reading into it things that were never stated.