Rear Rotor on Driver Side Has Lots of Black or Carbon like substance on it

[Ehill]

I've a 2003 Ford Explorer (V6-86,000 miles). I recently replaced my parking brake shoes/rear rotors and noticed yesterday that the left side rotor has a lot of black or carbon like substance on it. I checked the piston on the brake caliper and found no leaks. Also checked the brake fluid..........it's full. Any ideas what could be causing this? Does this means the parking brake shoe on this side of vehicle is not properly adjusted OR could it be a problem with the brake caliper sticking or whatever? The vehicle is stopping fine and there aren't any vibrations when applying the brakes. Appreciate any assistance!
Thanks

 

[wbrudi28]

is it much more than the other side? could just be the new pads/rotors.

also are they normal rotors? or slotted/drilled/dimpled? if so it will cause more brake dust.