I'm sure my EMS brethren will correct me but unless you're being treated by a military health care provider , no one is gonna believe that your blood type is XYZ, without typing it, nor will you be getting any blood products in the field anyway so I wouldn't bother putting that info on any sort of medic alert tag.

As for what to put on there, if you suffer from any sort of ailment that can/could leave you unconscious or otherwise unable to communicate, if you're allergic to any common medicines, if you are taking any prescription meds that could put you into a coma or unable to communicate, if you suffer from epilepsy, autism, parkinsons, hemophilia, cancer, asthma, hypertension, latex or any food allergies.

Lorazepam/Ativan is being used to treat seizures by EMS personnel. If you're taking it regularly already, I'd get a tag that stated this. Might theoretically stop an overdose in a worst case scenario. You don't need to spell out why you take it.