Yes, it darkens. It actually darkens a lot, but I'd say within two weeks it is mostly lightened back up. In that photo is has probably been three or four months since I applied Obe's LP, and they are about the same color as when un-oiled. If you constantly oil them, they will never lighten back up, but it isn't good for the leather to keep oiling it anyway.
I won't get a chance tonight, but in the next few days I'll try to oil them and take a photo of one oiled and one not. It's about time to re-treat them anyway because it has probably been close to a year since I did it last.