From King Saul to Uzzah, the Bible is full of examples of people who thought they could break one of God's commands based on what they had surmised or speculated another one might mean.
This is the position Jehovah's Witnesses find themselves in vis-à-vis blood.
Transfusion does not fall under the umbrella of any Biblical command and the JW claim that it does is speculative at its core.
Like King Saul and Uzzah, they have set aside plain and simple, black and white, clear and unambiguous commands based on what they have speculated that God might want. (But never actually said)
In the case of the JW's these are God's commands regarding the sanctity of life and the severe penalty associated with intentionally or unintentionally depriving another human being of it.