Blurry material for image in UMG widget

I have a UMG widget with a game menu. Behind that game menu is an image over the full screen. So that I can show a semi-transparent dark background behind the menu for example. Because that image is semi-transparent, I can see bits of the game behind the menu.

Now I would like to replace that dark background with a blur effect, for example as in the game menu of Half Life 2. There one can still see the game scene behind the menu, but as if looking through a dirty glas that makes the view blurry.

Is that effect somehow possible to achieve via a material that I would set for the image in the UMG widget, or do I have to do something else here?