I never was good about carrying the card, or even filling one in, when I was active. I did not carry one for a few years before I left.
I now proudly carry my Blood Donor Card.
I too want the best treatment, with the lowest risk for me, but as I believe this life that I now have is the only one I can be sure of, I am not going to risk it in any way that I can control.
I think carrying a JW No-Blood card is endangering your life unnecessarily, if it arises that you are compos mentis so as to make a choice you can refuse a BT without carrying the card or claiming to be of any religion, if you are un-conscious you are silly to cause any delay in needed treatment that may occur if the card is found, don't lose your life because you are a bit squeemish.