Along with a black discharge (looks like coffee grounds), odor and an increase in ear wax are indicators that your cat has mites (most cats get them at some point). There may be no observable signs. It can cause severe inflammation, so it needs to be treated. They can infest the ear or cat body and are contagious to other cats. If you treat your cat for mites and the ear does not improve, see your vet as your cat might have a yeast infection or a serious ear infection that an anti-mite treatment may aggravate. Check with your vet as to the best way to treat your pet for mites.