It is hard to tell from the pictures but it looks like the type of discoloring that is common when they are licking themselves repeatedly in the same area. The umbilical hernia doesn't look like it is affected or has anything going on. I would think more along skin irritation, contact allergy, or something along those lines. Probably worth making an appointment and having it checked out so it doesn't turn into something like a skin infection and to rule out anything more serious.
Have you gently cleaned the area?