Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear about your little one Toshi's black matter stuck to his skin around his mouth that can be removed but returns, and I understand your concern.
Does he look like the cat in the links below?
If these pictures look familiar then he likely has feline acne, which is technically increased sebum and plugged oil glands on his chin and around his mouth. Although this is more of a cosmetic thing in most cats they can get deep secondary infections so I do recommend treatment. Amazingly enough it doesn't appear to be painful to most cats.
I generally recommend topical treatment for acne.
Cleaning the affected area with Phisohex or Hibiclens once or twice daily and keeping the chin clean and dry is helpful. Phisohex or Hibiclens are over the counter facial cleansers used as an acne wash for people.
It flushes out the pores/hair follicles so is very helpful with feline acne too.
Just don't get it in the mouth as it can cause drooling and stomach upset.
I usually put a dab on a wet cotton ball or gauze square and clean the affected areas.
If he eats out of plastic bowls switch to glass or stainless steel.
And pay attention to the flavors of foods
he's eating as some cats with food allergies/sensitivities will get acne with a particular protein, for example fish. If that's the case avoid that protein.
If this condition becomes very inflamed your veterinarian may use cortisone to try and get things back to normal and sometimes oral antibiotics, but most of the time topical therapy is enough.
If it is getting severe I recommend looking for food allergies or other allergies as a reason for the flare.
I'd have to say food allergies are probably the most common trigger.
It can also be a sign of a flare up of demodex mites (which tend to worsen with a suppressed immune system) so it can be a sign that internal health isn't perfect.
Fungal infections very rarely cause this condition so it wouldn't be something that I would look for immediately, but a culture would be indicated if his chin isn't clearing.
If his acne doesn't clear with topical therapy, using metal or glass bowls and careful dietary control then antibiotics and a skin scraping as well as a fungal culture may be necessary.
Let me know if you have any further questions.