A small deep purple blister appeared on my penis head, just at the uretheral opening...it became raised in a few days and the subsided as if it was going to go away...it hasn't, and that was about 8 weeks ago...it looks like a normal small blister that is retreating (not raise since about week 2), and is about 2 mm wide...no wide fluid inside now or earlier...no pain, not sensitive, no other traditional STD symptoms

blister appeared about 5 days after unprotected sex

i can't find anything on the internet that matches these symptoms...what can it be?