Just to update you an my experiment. I mixed 50ml of malt vinegar with 400ml of water and dropped in a couple of pieces of black iron. I left the iron to rust of a few days until the mixture looked a rusty colour.
I took a piece of 15oz russet and an old belt (dyed black) and placed them in the solution for a time. Almost immediately there was a reaction with the new leather, small bubbles appeared and within the first 30 seconds it was evident a colouring was taking place.
At 2 minutes the new leather was a dark grey colour and the old belt was black.
At 3 minutes I removed both and the new leather was also black.
I rinsed both under running water and left them out on the bench to dry, the new leather has lightened slightly to a dark grey.
I then gave the new leather a couple of coats of Tandy Super Sheen and polished it up.
I'm pleased with the result, it's not as dark as some dyes but it shows the contrast nicely, a bit antique looking.
I did a cross section and it appears the reaction has gone about 1/2mm deep so no rubbing off with this method. See what you think.
Edit: 2nd photo, belt was dipped up to second hole from right.
I also slicked the edge by just dampening VERY slightly and rubbed up with a piece of old denim, it came up great with no need for edge coat. Anyone know if I should've edge coat to seal it?