One click said, "That looks like "smut" to me, a real deep layer of it.
Aluminium gets that way if bits of it in contact with steel are in an electrolytic (for de-rusting the steel) bath.
Zinc - the primary constituent of most "pot metals" - is quite electronegative relative to aluminium, almost as much as iron, and their process if it does that to pot metal probably also does it to steel-and-ally."

Yes, there are the two stainless pins for pivots, and I did not remove the hinge pin. Perhaps a factor.

is there any reversing the black?