saaaaame problem
i've been using cocoa butter on my elbows and knees for awhile. the darkness is still there, but its faded somewhat
i saw an interesting tip on the tyra show:
she said to put sunscreen all over the body at the beach/out in the sun, but put an extra amount on the elbows and knees. (ie. dont rub it in). you may look funny walking around with sunblock spots on your knees and elbows, but after a day in the sun, your skin should look more even.
i havent tried it yet, but it sounds like it might work
give it a shot!