From Jeff

1. Create Layer.
2. With magic wand tool…select outside of image.
3. Right click and select invert.
4. Go to top window bar and choose–>Select—>modify–>contract 2 pixels or 20 pixels or however big you want your blur to be.
5. Go to bottom of layer frame and click—> add layer mask button.
6. Go to filter—> blur—> Gaussian blur and adjust until desired effect. Probably 1 pixel more than what is selected in step 4.