Tip to prevent blackness of skin
Wash you skin with gram flour (baisan) instead of soap it safe you from harm chemicals and also fair you skin
Applying moisturizer would be the best way to prevent dark elbows and knees.
You can apply cocoa butter and honey mixture on knees and elbows.
Drinking water also clear skin complexion
Using SPF sunscreen block can go a long way in helping prevent skin darkness that may cause due to sun