It's possible that it's just dirt or grime on the iron. this does happen from time to time when covering with an iron that doesn't have a sock on it. I use my iron all the time without a sock as there are certain things that are just easier to do without a sock on the iron. And I do see small amounts of black "gunk" that will come off onto the covering. A quick fix for this is to get a bounce dryer sheet. Lay it on your work bench and rub the hot iron on it. This will clean off the bottom of the iron before putting it on the covering you are working with.

Ken