Painless Blister on Penis

Patient: Dear Doctor,I have a blister on the underside glan of my penis and I’ve been to see 2 doctors who don’t know what it is. I had swabs taken when I first saw it and they came back negative for warts and herpes. Every day it’s changed slightly. Sometimes puffy. Sometimes not. It doesn’t hurt or itch. I thought it may have been scabies so yesterday I treated myself for that, but I’d really like your help to determine what this could be. It’s so embarrassing and I really would like to get rid of it. Thanks!