It could be as simple as Feline Acne. There are mild cases which are most common, then (as pictured in the second link) it can progress into very severe.

I have one that from time to time gets a very mild outbreak. The stuff is black and kind of waxy, I just keep bathing the area with warm water, it's very difficult to remove and the sores if there are any are under that. If there is a sore I use a small amount of neosporin on a q-tip, to help it heal. It can sometimes take weeks though. You have to keep at it if that is indeed what it is.

http://www.valleypetnews.com/felineacne.htm

(more severe in this link)

http://www.fabcats.org/acneandstudtail.html