There are several possibilities: 1. When the tooth has plaque retention, it is easy to cause tooth decay and will form black-brown color, usually need to grind out and fill the tooth afterwards. 2. The darkening of the tooth root may be due to the formation of calculus caused by the plaque attached to the tooth surface. 3, the root of the tooth becomes black, may be due to metal braces easily diluted metal, resulting in the formation of pigmentation of the root of the tooth. 4, the root of the tooth becomes black may be due to the phenomenon of dental pulp necrosis, the tooth will form gray-black, choose to do dental root canal treatment, and do dental internal bleaching, can restore the color of the tooth.