Keep getting blisters on my penis, more when i masturbate, had all std test run and all negative what can it be?

Allergic? Chemical?: Too little detail to judge this. Herpes is the only STD that causes blister-like lesions or rash, but outbreaks aren't triggered by rubbing or friction, and occur no more often than once a month. (You probably masturbate more often.) Maybe an allergic or chemical reaction to a lubricant? Best plan is to see a doctor, preferably a dermatologist, within a day or two after onset of blisters.