One of the leading causes of dark skin on knees and elbows is the constant friction they might be receiving when you lean on your elbows or sit on your knees during work or play. The areas have fewer oil glands, which can cause the skin to dry out quickly. Also, dirt will accumulate in the dry areas, causing the dark spots. With a little care, you can get rid of the dark spots on your elbows and knees.
Wash the areas regularly to clean out the dirt that can accumulate in the areas. Exfoliate once a week.
Cut a lemon in half and sit with each elbow in each half for approximately 10 minutes. The lemon's acid should help gently break down the dark patches of skin. When done, wash with water.
Open up an aloe leaf and rub the the moist part on affected areas.
Moisturize twice a day. Use a cocoa butter or shea butter because these are thicker than regular lotions
Use sunscreen. Apply at least a 30 SPF sunscreen to knees and elbows while tanning to avoid the spots becoming even darker
Apply virgin coconut oil to help keep the skin moist and supple.
For extremely stubborn patches you can try skin bleaching lotions that can be found in most drugstores.