Ouch, I had a similar experience the other day. I took off some old seatcovers that had a rip down the center, as i took them off i noticed one of my douchebag friends had also done the same thing hahahaha and it was hardened and black on the back of my tan seats. i used some interior cleaner from pep boys and my finger nails

and i got most of it off...im gonna do it again later today...ill see if i can find the name of the stuff i used, i know its in a blue bottle and its under 5 bucks.