it's just carbon residue left from the combustion process. all engines have them and there's nothing worng with it. the carbon will clean right off by means of scrubbing, blasting, washing, etc. be sure to get it real clean. don't want loose carbon wandering around between ring grooves. there should be nothing worng with the piston unless there is noticeable deformations.
by the sounds of your question, you seem new to engine rebuilding. better take it one step at a time and be sure to look up and ask questions.