If those dark areas you want to get rid of are fairly consistent in color this is how I'd do it using Paintshop Pro. Not sure if you can do the same in Photoshop or not.
Set your background color to match the color of the shaded area, use the eyedropper to grab the color.
Then Paste As Transparent Selection.
The pasted floating layer will have the dark areas removed and transparent. Copy it while it's still a floating layer.
Now paste it on a clean background layer and your dark areas should be gone.
There may be a few stray pixels if the dark areas were not all one color across all pixels. Just repeat the process by setting the background color to match those stray pixels.
The only times this hasn't worked for me was when the there were too many pixels with different colors that I needed to get rid of. But from the looks of that pic it seems like you wouldn't have that problem.