People have a right to believe what they want even if it makes no sense at all and medical professionals generally respect that. Faith by it's very nature is not rational, so attempting to deconstruct a purely faith-based proposition with scientific observation and formal logic is a complete waste of time.
However, I would argue that Jehovah's Witnesses are a special case. Jehovah's Witnesses not only eschew the notion of blind religious faith, they ridicule it. They believe the world we perceive is a real one and reject systems like Christian Science or Hinduism which teach that only the mind or spirit is real. Jehovah's Witnesses believe what they believe, not as matters of faith but because they believe that the evidence when honestly examined leads to their belief as an inescapable conclusion. That's the whole basis for the "Preaching Work" and the notion that ours is a time of judgment. As Penton observed, they are rationalists par-excellence.
Their stance on blood is no exception. It is not an ontological argument. It is a mechanical argument of physical equivalency between the consumption of blood and the transfusion of blood. Because of that, it's inherently technical in nature inasmuch as the adherent is obligated to bridge the gap between religion and science every time they attempt to state that equivalency.