I'm dissapointed that, so far, the majority would actually violate another person's choice as to their own medical wishes. How is that any different than someone else violating our own choice to take a blood transfusion on the basis that they do not share our belief? We may not like it, but it is still the other person's choice to choose their own medical treatment, even if it means them possibly dying from it. Free will includes the ability to choose how we live or die, and to take that away from some one is the same as taking away their right to be disconnected from a life-prolonging machine if they so choose.
But going back to the original scenario, if one is uncomfortable signing someone's no blood document, then they shouldn't. Simple as that. If one were to have signed and then later changed their mind about carrying out the other person's wishes, that one should inform the other person involved so that they can sign a new version of the form in order to take you out of the equation. It's about mutual respect.