How many times have you oiled your cinch? Okay, now how many times have you oiled your saddle?
Oiling is just adding moisture to the leather, which darkens the leather considerably. But as it dries, it will lighten up again. My saddle is Havanna brown, but after 35 years of oiling and use, it looks almost black. Give it time, and keep oiling. Eventually, the cinch will catch up to the saddle.
There are products out there to use to darken leather, I would call Tandys Leather Factory or Springfield Leather and see what they have to recommend. You could even use an oil based dye or stain to artificially darken it too, but unless you know how to cut the dyes or stains, I don't recommend it (especially after it has been oiled).