CSS Blurry Shimmer Effect

Imagine box-shadow but for a blur effect, where the backdrop of an element is blurred around that element, gradually decreasing the blur’s strength. I came up with the idea while trying to improve the contrast of a popup over a dark area where a box-shadow for the popup won’t make much sense, design-wise. I then thought, well, what other ways might create a good contrast effect? And so suddenly, the idea of a gradual blur effect around the object came to me.
See the Pen Faded Outer B