Something dark gray coming off of sheet pan after scouring
It is oxidised aluminium (Al2O3). This is formed by oxidising with the oxygen in the air to form the aluminium oxide. Al2O3 forms a very thin (4 nm) impermeable layer on your Al, which prevents further oxidation and appears as a dull grey on the surface. Polished Al is shiny and can be highly reflective.
Al2O3 appears as a dark grey/black colour when you scrape it and some of the underlying Al off the sheet with your steel wool. It is likely that your &quot;steel&quot; sheets are actually aluminized steel, as the steel itself is not rust-resistant. The aluminium coating on the steel prevents corrosion from the air by forming the aforementioned oxide layer.