My friend used to have a black saddle that had faded loads and she was recommended to use fabric dye, she applied to the area affected left it to dry, washed/rinsed with a damp sponge afterwards to get the excess dye off and then waxed over the top to trap it in......pretty sure it never stained her jodhs
Speak to a saddler first or google it you may find out more, i believe you can buy leather dye's too. Someone on your other post suggested neats foot oil which darkens things that might work.