Probably just soot from the engine. Not really a pretty sight. I'm sure it can be easily wiped/washed off. I'm quite sure someone here can give you a better answer.
It is just soot, probably from the APU, as others have stated.
But.......Its not easily wiped/washed off. And when you get it on your skin, you might as well just wear off a layer of skin. It does not want to come off, no matter what you try. My wife is always accusing me off not washing my hands. Yeah i just spent 1/2 hour taking a layer of skin off with a brilo pad, but I didn't wash my hands.