The hospital wouldn't trust a bracelet anyway, as they have to type everyone before they can give blood, to make sure there aren't any antibodies in your blood that will react with the donor blood. I have given many a unit, and they always do a crossmatch with each individual unit against your blood, trying to insure there will not be a reaction.
If you are bleeding and the doctor thinks you cannot wait the 15 min or so until the crossmatch gets back, they will give O negative. But this is extremely rare to give uncrossmatched blood, I have only done it once in my 12+ years as a nurse.
So I wouldn't waste my money on the jewelry or a tat( unless you just want a tat with your blood type incorporated into it.
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