Hey, thanks so much! I did what you said except that when I copied the shape with properties, the opacity was red and locked, so I had to do a regular copy. But… when I applied a Gaussian blur to the adjustment layer, it just brought out the dark tones of the copied shape instead of blurring the background image below. I also tried applying the effect to the copied shape layer, but that didn’t work, either. Maybe I did something wrong there.
What I ended up doing that worked perfectly was duplicating the shape layer and setting the rectangle’s opacity to 100, duplicating the background image and using that instead of an adjustment layer, below the copied shape, set to alpha matte, and then applying the blur to the duplicated background image layer.
Thanks for pointing me in the right direction ☺