Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Most likely Blacky has a staoh infection in these areas that is causing this skin infection and the odor. The staph bacteria is normal flora on their skin and when it over grow, it causes this hair loss and odor. The best way to treat this, if you can get near Blackie is to apply Betadine to this area every 6 hrs. This can be purchased over the counter at the chemist and will dry out this area so teh bacteria cannot grow. This is a common issue in some older cats and is called Feline Acne when on the chin.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta