I'm 99% sure I have that enabled, I think it's just called blur or is easily found by searching blur in the general os search bar. It might even be under "desktop effects > window management"
I posted this as a quick example but I've seen people have it blue in a lot of places, I including the windows that are opaque in this shot
Here we go, within the first 30 seconds of this video it'll showcase it
Surgery advanced