> For proof of this, check out your aluminum baking sheets. If they’ve been used, they’re almost certainly covered in a shiny black substance that’s a complete pain in the ass to remove, even with abrasives. That’s essentially what’s on your cast iron.

I'd never made this link before, thanks for pointing it out! We have plenty of baking sheets like this and I thought it was a form of black rust or something but your explanation makes so much more sense.