About a month ago, I bought a used 2009 Stumpjumper FSR similar to this. It was not used heavily and remains in great condition. Recently however, after medium to long rides, a black residue is left on the damper body of the rear shock, (A Specialized AFR Brain Shock 7.25x1.75").
If I try to remove the residue with my finger, it smears slightly across the damper body but doesn't seem to stain my finger like oil would. It's also fairly difficult to fully clean off. According to the seller, the bike received a comprehensive tune-up about a year ago including a re-grease/oil of the fork and shock and hadn't been ridden after that.
What might this residue be? And could it just be that the shock requires some new grease on the inside of the air can, or would it be advised to do a more complete air can service and replace the O-rings? (if so, what rebuild kit would work for this shock?)