This is an example of the Blur Up implementation.

Here is the explanation of how it works

"The (ingenious) solution was to return a tiny image (around 40 pixels wide) and then scale that tiny image up whilst applying a gaussian blur. This instantly shows a background that looks aesthetically pleasing, and gives a preview of how the cover image would look. The actual cover image could then be loaded in the background in good time, and smoothly switched in. Smart thinking!" - CSS Tricks

You can check a demo in this link https://gracious-jennings-0e5ca9.netlify.app.