I have not visited a doctor about this problem because
It may not be due to infection as you don't have any other symptoms.
I can understand your concern. It may be due to vigorous act of masturbation or injury. As per your history, you don't have any other symptoms which rule out chances of infection.
Please let me know that whether you had undergone any surgery in the past, do you have history of injury to private parts, scrotum or over back region? Kindly follow up with detailed history.
I think black fuzzy stuff may be due to clotted blood caused due to injury or vigorous act of masturbation. I will suggest you to do color doppler ultrasound of private area including scrotum after urologist consultation.
Hope I have answered your query. Please let me know if I can assist you further. Thank you.
Dr. Shyam Kale
Family and General physician.