These are probably "age spots", or sun spots, and can be removed by nightly application of a bleaching cream one can buy in the drug stores all over. The process is slow, and patience is important, but they work! I think Aveeno makes one, but wouldn't swear to it.
There is one home remedy for the dark areas on the elbows and finger joints. Take a lemon and cut it in to 2 halves. Press the lemon and empty its juice. The empty half of the lemon rub on the elbow and finger joints. You can also add a bit of salt in the lemon. This can work to treat dark area on the elbow and finger joints.