Well, the first thing is to make sure that you are definitely doing what I recommended in my previous post (See Post: Is shaving making the skin on your face darker?).
Then after that, you could try a skin brightener… No, just kidding! The results of skin brighteners are extremely varied. Do I really need to mention Sammy Sosa? ~> (click here!)
The safest and least invasive option is to be patient with your skin; in time it will lighten. But while you wait, incorporate an exfoliation regimen that is focused on your darkened areas. As I’ve mentioned before, exfoliating gently removes the dead skin cells to free up younger and presumably healthier layers of cells that are working their way to the surface. Trust me, if it worked for me, it will for you!
Best of luck!