Does anyone know what this is?

Hi all,

My cat suddenly has a very dark/almost black ring around his nose. He has gotten a much lighter one in the past that has gone away but this is really dark and Im concerned.  Does anyone know what this is? Just a little background on him..he was a partially feral/stray cat that has been living indoors as my pet for almost 6 months. He is felv+ and also just tested +4 positive for Bartonella. He is currently taking azithromycin and prednisolone. He has stomatitis that is pretty bad.  He is playful and eating ok. Thanks so much for any input!!

 Have you tried washing it off?  He has some black gunk around his eyes also.  Could be simply a bunch of dust that has accumulated in damp areas.  If it washes off with a cotton ball and mild shampoo or dish soap and the skin underneath is pink, not oozing or red, then that is all it is.  Kittens get this from milk around their nose picking up dust.  Most cats wash their face but kittens and some ill cats do not.

Thank you both so much for your responses. I was seriously about to make a vet appt thinking he had something else wrong with him but I took a baby wipe to his nose and its coming off! Yucky! Thank you so much.