The dark skin on your elbows and knees is quite common. There could be several reasons for the cause of the skin appearing darker than the other parts of your body. Some of the more common reasons for dark elbows and knees are over exposure to sun, dryness, inadequate skin care, and heredity.  This may not be a very serious issue but it definitely causes embarrassment for womenfolk. Hence, it is helpful to make the skin tone lighter in these areas so that you can wear that favorite dress without any embarrassment next time you are going out.