You can say "their choice was not to receive blood but my choice is to allow it" and that is your choice to make if they entrusted that situation to you. If they didn't want you to be able to make the choice then they shouldn't have asked you to.
There is no reason for them to be unhappy because receiving a blood transfusion while unconscious was not their choice so they have not violated any commandment anymore than someone being drugged and raped has been immoral.
If they or their religion can't differentiation that then it's their problem - you have done the right thing by saving their life and giving them the options to make future choices including how to handle the difficult choice you were put in a position to make.
Simon: That's pretty much what I will go through if my wife needs blood. But I have to disagree with "There is no reason for them to be unhappy because receiving a blood transfusion while unconscious was not their choice so they have not violated any commandment..." That frigging cult makes such a big deal of this that it doesn't play out that way. With a child, I believe it does. Deep down, every JW parent hopes the court forces the issue. But for hardcore adult dubs coming back from unconsciousness to discover they were given blood, the cult makes them feel that they have to be angry, feel like they failed.