My teeth (upper and lower incisors and molars) are getting blackened. Actually black! Which does not come off even after brushing. No history of tobacco consumption, smoking,tea or coffee. I've got scaling about 5 times. After each scaling it gets cleared only to recurrently come back. The last session was about 4 years ago. The toothpaste I use is either Colgate or pepsodent. I brush twice a day. Other than the above mentioned teeth, rest all are so amazingly white!