Hello,
Thanks for the query.
Although a blister on penis can be caused by simple frictional force 
 like doing a rough sex or rough masturbation also but in general blisters on penis indicate some type of underlying infection or illness.
If you are a sexually active person,then tjis could indicate a sign of underlying sexually transmitted disease like genital herpes or genital wart or syphilis etc.
It could be due to a simple yeast infection also.
The best would be to consult a doctor and get yourself investigated and screened properly.
Treatment would depend upon finding the cause.
Till then don't engage in any masturbation or sexual activity.
I hope it helps however you may revert to me for any further query.
Best of luck
Thanks

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