The knee darkened mainly the following possibilities: 1, friction caused by: this situation is mainly repeated friction caused by the skin protective response, the skin by thickening, hardening to counteract the damage caused by friction on the skin, while there will be darkening skin tone, in the skin to reduce friction can slowly fade, you can use topical moisturizing cream. 2, post-inflammatory pigmentation: sometimes eczema, dermatitis recovery can occur after the onset of pigmentation The actual fact that this situation can be self-healing, most people can slowly fade in color. 3, part of the disease caused by: such as fixed drug rash will also appear similar performance, generally fixed drug rash is mainly antipyretic painkillers, antibiotics, especially sulfonamides caused by the main skin surface round or oval patches, after the drug metabolism most people can slowly fade. Therefore, if you find that your knee has turned black, you must first clarify the cause of the disease, and then treat it according to the specific cause.