To make this type of blackening you will need some plain steel wool and apple cider vinegar, as well as an old pot. Take enough of the steel wool to fill a quart jar and then place the steel wool in the pot, next, pour a quart of pure apple cider over the steel wool and heat it to near boiling point. When it is cool, put it in a glass or plastic container and leave it to sit for about a week or so.
It should then be reheated again and when cool poured back into the container and left for another week or so.
Lastly, strain off the liquid into a non metallic container. To use it, apply several coats, which will go on as a sort of light grey colour, but as it sets into the leather with each coat, it will become a nice black that will not rub off.
If you have ever left a piece of steel wool or bare metal on damp leather it will leave a black mark that will not come off, even with bleaches or other chemical cleaners.