blisters can mean a number of things and can be any number of stds.
while the blister is there go to your local public health center to get it swabbed.
dial 411 and ask for the # of the public health department for your county.
when you call ask about a free full STD screening and tell them what is going on with your penis. if it goes away dont forget about it. it could be something else that could render your sterile or kill you (syphillis) even if you wore a condom, a condom doesnt totally protect against herpes because the areas that can be infected are not totally covered. also if you recieved any oral sex without the condom you can catch whatever from a persons mouth that you can catch from thier genitals.
they are both mucous membranes. even a hand can carry herpes (herpes whitlow of the finger)
good luck! go get checked out!