We wish to darken the borders of an image in a smooth manner. This can be achieved in probably a hundred thousand ways or more, but I shall present the Quick-Mask-Mode-way.

1. Select an area using the Marquee Tool (M).

   

2. Enter Quick Mask Mode (Q).

   

3. To edit the mask you can use any brush, shape, text, filters, etc. Let’s apply a Gaussian blur filter.

   

4. Invert the mask (Ctr+I).

   

5. Exit Quick Mask Mode (Q).

   

6. Edit the masked image to your liking. Open the Levels dialog (Ctr+L) and move the black point slider and the midtone slider up.

   

Sweet.

Btw: the picture was taken at the Santa Barbara City College.