Seems like a classic thing many of us have experienced . Due to the not obvious way that stainless steel protects itself. Stainless steel actually protects itself with a very thin layer of chromium oxide that prevents further oxidation. Not by not forming an oxide as you may think. The oxide is very thin, a few microns , It Is not really visible, forms very quickly BUT is not very wear resistant. You are supposed to AVOID wearing on it. It will form again instantly. What you experienced will happen. Seemingly wear product emerging from nothing. The colour of the wear product will depend on what wears it of reasonably and present dirt etc. Black seems reasonable. What you are doing is very very slowly wearing on the stainless pulley. To prevent this you could use a material with hard surface oxide instead. Anodized aluminium should work well for that.