Use a clean needle (cleaned with alcohol), pierce in two locations, and press to empty the blister. If it fills again, repeat the process, in a few days it will be dry and form a callous (that will go away after a while).
Tested on upright bass blister's, haven't seen a better way to deal with it.
Actually, there's no skin on it anymore, so nothing to pierce. I've noticed the blister (or what was left of it) when I was way past the "bubble stage". I guess I overdid it with the screwdriver.
So now I'm just hoping that it will dry soon enough, but that would have been great to have a magic trick to speed up the process ;)
Thanks a lot anyway ! :)
Generalistic instance that is art friendly. Goofy people are welcome !