Got me, I'm stumped. On one hand I would say, mask all around the area and just spray bomb it black. Although that sounds crude, it might be a good temporary fix.... Say you want to take some good pics of it.
On the other hand, I would say get a new liner.
Really, it's just dirt residue from over the years. I don't know if a shop vac would help get the dust and crap out of it or not. I would suggest if you try this to do it in an area that is in the back and not real noticeable, and see how it goes.
Let us know what you decide to do.