Pitch Blend, Minks Oil and Neetsfoot Oil will darken it some, if you are looking to change the color to a darker color, then your best bet is Fiebing's Leather Dye. It comes in various colors, you can get the dark brown to go almost black with a few coats. After putting the dye on, wipe it dry, and then you can restore the water proofing with some Montana Pitch Blend.
Permanent markers and wood stain will work also, but can run the chance of really drying the leather out, plus, in wood stains there are petroleum products that can leave a smell that is hard or impossible to get rid of...