My wife recently accidentally tore open a couple of finger blisters. She used some Burt's Bees Res-Q Ointment on them and they healed up much quicker than I'd expect. They were identifiable, but reasonably tough and painfree in a couple of days. I don't know that it would work on unopened blisters or that they'd then be strong enough to hold up to string bending, but it's probably worth a shot.