yup, steel wool, iron fillings ect, any sort of iron. This was used not only as a leather dye but a wood dye as well on rifle stocks way back in the day. early 1800s.
Steel Wool + Vinegar ==>Iron (II) acetate (aka Ferrous Acetate)
Iron (II) acetate + Tannic Acid ==> somehow makes black leather
Other homeade dyes are Tabacco dye, Walnut husk made dyes, ect. Leather has been around alot longer then the fancy factories we got that will make the dyes for us.