Black film on the inside of gas tank

I recently dumped old gas out of my tank and in the process held it after with my finger in the lip of the open tank. My finger came out black, like with black grease. No reason for that I can think of. The inside is actually white with some sealant coat -- which is starting to peel by the way, though not plugging the wire mesh over the taps or getting into the mikuni carb. Now, the film came from the top of the tank by the gas cap.

Deteriorated cap gasket?

At work right now. I'll check when I get home. But the thin black greasy film would have had to migrate at least to where my fingers could reach. Could be. I'll check.