when I scratch the tip of her ears, it looks like dirt is coming off
If you live in a particularly cold climate, the edges of your pups ears may have a touch of frostbite. That would explain the dark skin cells that are sloughing off when you scratch or rub at her ear tips. Try to protect her ears in the cold. Be gentle when you rub the ends. Depending on the severity, consult your Veterinarian, antibiotics may be needed.