Diagnosis
That's grease or carbon dust. It's coming from a worn drive belt (the belt that turns the drum) rubbing inside the cabinet, or, more likely on this model, from the drum support rollers or their axles which require lubrication.
How to Fix It
- Unplug the dryer and remove the front panel (requires removing the top panel first, then several Torx screws around the door opening).
- Inspect the large drive belt for signs of fraying, glazing, or black dust around it.
- Look at the two or three white plastic drum support rollers at the bottom rear of the drum. Spin them by hand. If they squeak, feel gritty, or have visible black residue on the roller or its metal shaft, they are the source.
- Replace any worn rollers and clean the metal shafts with rubbing alcohol before installing new ones.
If That Didn't Work
The black marks could be mold from chronic moisture if you never run the 'Empty Dry' or 'Drum Clean' cycles, or if the drain system is faulty and leaving the drum damp. Clean the drum with a vinegar solution.
Heads Up
Don't just wipe the drum and continue. The contamination will recur until you replace the failing mechanical part. Running it with dry, unlubricated rollers can score the drum's glide path, necessitating a full drum replacement.