How to make an effect on any object that will blur the background?

Question

Design

I need to make an effect in illustrator, or in any other editor, let's say on a cube with a transparency of 50% and everything behind the cube will be blurred, and everything that is not in the cube is not blurry, how to make such an effect, please tell me!!!

Answer 1

In Photoshop (your "cube" and background should be layered):
1. Duplicate the background (let's call the new layer "non-fog"). Returning to the backdrop, blur it as you need.
2. Add a mask to the "non-fog" (with the layer selected - "Layer -> Layer Mask -> Reveal All").
2. Select the contours of your "cube", i.e.
the area that should become blurry (Ctrl + LMB on the layer icon with the "cube"), go to the "non-fog" mask (it is important not to confuse the layer mask with the picture of the same layer) and fill the selected area with black. image in the "non-fog" layer, the mask area (filled with black) will be hidden, through which part of the blurry backdrop will be visible.
The opposite effect can be achieved by inverting/hiding the mask effect and/or changing the layer order.