How To Cure Blisters In Child?

My 8 month old daughter has had a raised mark on her arm for a few months now. It looks like a dome sized scar but when she gets warm or we put any kind of medication on it, it blisters then pops. The doctor has given us 3 different medications for it and it only makes it blister. It never bothers her even when the blister pops and it always goes back to a raised scar. Do you have any idea what this could be?