I think they're an excellent idea, especially for unusual allergies or conditions such as diabetes. Don't worry about the blood type, though... no one is going to give you blood without doing a proper cross-match, unless it's an utter emergency related to a severe bleed. And in that case, they use O-negative blood until they can get a cross-match done (which can be done in minutes by a good lab). So use the space for the other info, or cheery little messages such as the one Spaw suggested.