I have no idea what will cause it but my seats did have a black stain. I "think" dirt and grime gets into the pores of the vinyl and turn a black color.

One year for christmas I got a car cleaning kit as a present. Inside the kit was vinyl seat cleaner. It was a foam that came in a spray can. Spray in on and let it set for a few minutes then wipe it off. The harder you scrub the more clean they get. It is best to do small sections at a time. It took me about 1 hour to do two bucket seats but they looked like new after I was done. I have seen the cleaner sold by itself at wal-Mart. It comes in a yellow and red colored spray can. I cannot remember the name but it is some good stuff if you spends some time scrubbing with a rag.