Bits of steel will absolutely stain a liner. Usually the stains will be orange, not black, but black is possible. If you suspect iron from that source, Iron Out offers a better test.

Buy a bottle of Iron Out (detergent section of Walmart, etc) and put 1/2 cup in a snack bag. Open the bag against a stained area, so the Iron Out gets wet, but is not washed away. Again, 5 minutes of contact should be sufficient to produce visible change.

Test grit with the snack baggie method, but use sodium bisulfate (dry acid or pH Minus). Wear plastic gloves.