Hello lybrate-user, the black colour of your teeth which you are talking about is probably due to the calculus deposits in that area which with time gets stained depending upon the foods you take. It occurs because of improper brushing in that region regardless of what toothpaste you use. So, the most important thing is your brushing technique which you need to improve. Have some look over the youtube videos, you will get to know the proper method. Once the plaque gets calcified, it can not be removed by normal brushing so do not try removing it by brushing hardly, it will only damage your teeth more and will cause abrasion leading to senstivity. So, here?s my suggestion to you:
1) firstly, to get it removed, you?ll have to visit your nearby dentist and get your scaling done. 2) once done with it, do regular warm saline rinses for atleast a week or two. 3) and for a few weeks try using chlorhexideine mouthwash, before going to sleep. 4) last but not least, improve your brushing technique otherwise there are always chances that it will happen again in sometime.