I believe that's what I did. Here is a screen capture of the image and the layers panel. The part that's selected is the darker colored rocks in the middle and leading off to the right edge of the frame. What you see is an almost final version of the image. At this point my interest has shifted to just seeing if there's another way to do this, like what you've suggested.
Bill

Here it is with the Color Fill layer:

Here it is after the Image > Adjustment > Invert. What concerns me is that the background are covered with a much lighter color of gray than others areas. This area was not part of the initial selection, but seems to be acting like it was. I expected it to be the same gray color as other areas.

After applying the Gausian Blur there's a small area outside the selected area that is now visible.

And now with opacity set at 25% here's what resulsts. The background rocks / ridge is now much lighter than it should be.

Hope this gives you some "diagnostic" information.
Bill
Here's another way you could do this using a Color Fill layer: With your selection active, make a new Color Fill layer. In Blending Options, set Fill Opacity to zero percent. Add a Stroke, with Position set to Inside, Fill Type set to Gradient, Style set to Shape Burst, and a Black to Transparent gradient.


[EDIT]
Better than a Stroke, use Inner Glow, with color set to black, Blend Mode set to Multiply, and Source set to Edge.
