There are several reasons for black joints: 1. Black joints are mainly due to friction in daily life, repeated mechanical friction leads to thickening of the keratin layer of the skin, followed by pigmentation, especially in the elbow joints and knee joints. 2. 3.It is possible that the individual parts of the skin have folds, and when the joints are relaxed, dirt can lurk in the folds and look dark over time. 4.It is also possible that the vitamin B deficiency caused by vitamin B deficiency, plus long-term exposure of the skin, due to light exposure, chemical stimulation or the body itself The dryness of the skin and other factors lead to.