Hello,
Welcome to healthcare magic

Saw the picture you sent. I could not place where exactly the location of this blister is on the penis. Can you upload another one with a farther shot ?
Is there any other smaller vesicles near the big one ?
Is it painful or itchy ?
What are the medicines you are taking for blood pressure, diabetes and arthritis ?
Do you have skin lesions anywhere else in the body ?

Kindly give me the above details so that I can answer your query better.
Take care

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Hello,
Welcome back
You did not give details as requested.
If it is painful it could be herpes even though it is rather rare to have only a single lesion.
If it is itchy it could be an allergic contact dermatitis
It could even be a friction blister if you have indulged in vigorous sexual intercourse or masturbation.
It could even be a drug rash, some medicines like furosemide given for hypertension can cause blisters.
It is not a classical skin disease which can be diagnosed only by a picture. Examination of rest of the body and history also is important to accurately diagnose.
You may apply an antibiotic cream over the blister thrice a day.

Hope this helped
Take care

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Hello,
Welcome back
Friction due to sexual intercourse especially if vagina is dry ( due to inadequate arousal or due to dryness because of menopause) can cause a friction blister which can look like this.
You may puncture the blister with a sterile needle and drain fluid and apply antibiotic cream like mupirocin 2% or fusidic acid or neosporin thrice a day.
This should be enough.
Hope this helped
Take care

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