... but, they fail to take note of one important detail.
I believe that if we are going to have any chance of reasoning with a JW about blood, this is the place we need to begin.
Don't try to convince them that it was only a dietary law. It wasn't, and they will never go along with it.
Don't tell them that saving a life is more important than obeying a law, even a seemingly trivial one. They take pride in obedience.
We need to find common ground with JW friends or relatives if we are going to help them to reason. For the sake of argument we can concede the following;
1. Blood was sacred under The Law.
2. Blood represents life.
3. It was a capital offense to use blood for any purpose other than to offer it as a sacrifice on the altar.
4. All of the restrictions about blood are just as binding on Christians as they were on OT Jews.
It doesn't matter if you agree with all four of these points or not. They believe them, and we can concede all of them and start from there.
Food
When an Israelite killed an animal for food he was required to acknowledge that it's life belonged to god. By pouring out it's blood on the ground the life of the animal was symbolically returned to the life-giver.
Sacrifice
The principle behind all of the sacrifices under the Law was vicarious punishment. The penitent was acknowledging that they deserved to die for their sins, but god was willing to accept the life of an animal in his place. The blood that was poured out on the altar represented the life of the sacrificial animal being offered to god.
In both cases blood was only sacred once a life had been taken.
If an Israelite farmer found an animal "already dead" he was free to eat it with impunity. - Lev.11:38,39. Once an animal has been dead for more than a few minutes it is physically impossible to bleed it, so under these circumstances the Law is giving permission to eat unbled meat.
If an Israelite was to bleed an animal without killing it - as the Maasai do - and take the blood to the altar, the blood would have no sacrificial value for the simple reason that no life had been taken.
This is the important detail that the Watchtower have overlooked.
Blood is not intrinsically sacred; it is only sacred insofar as it represents a life that has been taken.
In the case of a transfusion no life has been taken and therefore the blood is not sacred. It can be used to sustain life, just as the Israelite could eat the unbled meat of an animal found "already dead".
I am convinced that the above approach has potential to help some JWs to question the blood doctrine. It is based on the fuller explanation at this link... where I also discuss the context of Acts 15.
If anybody would like to play Devils' Advocate on this I would be interested to see how it holds up.