A stain that transfers from dark jeans to a leather car seat is known as a "dye transfer." The dye from the jeans rubbed up against the leather, discolouring the surface. According to the website How to Clean Stuff, you can still get your seats back to their original condition if the stain doesn't penetrate below the surface. The key is to use a cleaning agent specially formulated to remove dye stains from leather.
Dilute the leather cleaner in water according to the manufacturer's instructions. Mix the cleaner and water together well in a spray bottle. Shake the bottle vigorously.
Spray a tiny corner of your cloth in the solution and test it on a small area of the leather. Some leather cleaners designed to remove dye can cause bleaching or discolouration. If you don't see any bleaching or discolouration, proceed.
Spray the cloth twice and scrub vigorously at the stain. Repeat until the stain vanishes.
Dab a quarter-sized amount of leather conditioner onto a clean soft cloth. Rub where the stain was and the surrounding areas.
Let the conditioner set and penetrate deep into the leather for about 10 minutes. Buff it dry with a clean cloth.