a clear non painful open blister on the shaft of my penis

A blister usually forms because the outer layer of the skin has become damaged.
Blisters can also form as the result of certain medical conditions.

Your blister may be Herpes, irritation or injury. You need to let your physician check it out, and it's a good idea to abstain from sex until you know.

A blister is never something to ignore. Healthy genitals should be free of sores. A simple culture could determine the condition.

Penis herpes, Irritation or injury causes...