Here's a fun little video!
Apologies for the dogs barking in the background (Parker is jealous).
The liquid is vinegaroon! It's made by mixing white vinegar with metal, usually steel wool because it corroded faster but this batch I made with some old rusty drill bits. Let the metal sit in the vinegar for awhile (a few days is usually good, longer than that and you start getting iron filings dropping out of the solution).
Vinegaroon only interacts with tannins! That's why there isn't much of a reaction with the paper but the leather turns black. It does the same thing with any wood high in tannins too!
This is what I've been using with the dog collars I've been making to turn the edges black and any black work I need in the tooled areas.
It's one of the oldest ways to turn wood or leather black.