What does "brightened" mean?
I am using the word "brightened" to mean you have brighter teeth. It's not to say you will have better teeth or that you will never get black dots or black streaks in your teeth. It's simply because you have stronger teeth that have more dentine. Dental dentine is made up of tooth enamel (and other tooth parts) and bone that surrounds the tooth. When tooth enamel starts to wear away, it weakens, and the teeth lose their dentine (and teeth come into less contact with the enamel).
I have a lot of tooth enamel, and if I ever had a hard time with my teeth or felt that they were too white or blue or red or something like that, I was probably going to lose a tooth. If your teeth look white, blue or black, chances are that your teeth were not made to withstand the pressure and stress of daily brushing. Your teeth, especially those that come in contact with enamel, can be overworked and have a hard time doing their job of keeping you healthy. If your teeth don't have enough dentine, you are likely to lose some teeth, and if you do lose one, it will likely be in the front of your mouth or on the upper sides of your teeth.