I have. It's very subtle and lightens as it dries. I usually put 2-3 coats on the first day, then a coat every 3 days until it's the color I want it. Letting it dry in the sun will help darken leather a little quicker.
Using olive oil on tack and letting it dry in the sun will also darken the leather.
Darkening oil only works on leather that is well cleaned without a high shine. If there's too much "gloss" on leather before you start, not as much of the oil will soak in.