First and foremost, you cannot question anything against the Watchtower without getting either disfellowshipped or marked.
What I suggest you do is to get a copy of their old anti-blood booklet, Blood, Medicine and the Law of God, where they had the original teaching and use it against them with the simple statement that they were consistent back then. I know you'll get the usual "old light" response but you might have better luck if you have them read it and cause some dissonance in their perception of the doctrine. Don't know if that might actually work but it's worth a try.