You haven't mentioned exactly after how much time you get those stains back.as scaling may be required every 6months depending on patient's condition.there is no harm in that too. Black staining can occur also because any supplements you are taking(medicines)on a regular basis ,or may be prolonged use of mouthwash .please check on these causes also.dont be upset your situation will get better once you get a proper consultation with a doctor.visit your doctor and consult by giving proper history about your health.and after scaling you got to maintain the situation by following instructions of your doctor.