Depending on what the blemish was caused by, you may need to use a leather rejuvenator type product. I would start with a towel with just warm water to start, add some mild lotion-free hand soap if it doesn't come off.
I use Griot's Leather Care and Leather Care Spray products which I've had great results bringing older leather back to life. I use it more often on my briefs (hand bag for work) vs my car seats, but works great on car stuff too, of course.