Black boil in head of penis

Patient: I am a male aged 40.i am orthopaedically handicapped.i am not married. i didnt have sex with maleor female till now. 10 years back i had a black boil like thing in thehead of my penis near the hole where the urine comes. i have no painor no pus coming in that. i have no itching or no irritation. i amdoing masturbation for the past 20 month back only i pulledmy foreskin back. for the past 20 years i didnt tried to pull it backwhile masturbating. i have no diabetes or blood pressure. what is this?is this a std or cancer ?