The most common cause of black debris in a cat's ears is ear mites, according to the ASPCA. Ear mites leave a debris that resembles coffee grounds and can be seen where ever the cat rests or scratches.
Ear mites are highly contagious and potentially cause a variety of skin and ear infections. A cat with ear mites may scratch and rub the ears excessively, shake the head and experience hair loss or sores around the ears and neck. Ear mites are more common in kittens and cats who stay outdoors. A cat with a suspected case of ear mites should be seen by a vet immediately.