This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
An inexpensive and effective way to blacken tan leather is as follows: The leather is first rubbed with a 10-percent solution of tannic acid, which may be purchased at any drug store. This treatment should be applied and the leather well dried. It should be rubbed with a cloth hard enough to produce a polish, then apply a 10-percent solution of iron sulphate. A chemical reaction takes place as the last solution is rubbed into the leather, making it black. After this is dry, the leather can be polished in the usual way.