How to make a blur of content for a fixed block size?

Hello!
How to make blur the content (not just images and text, etc.) per unit, which is fixed? That is, you need to blur only part of the page that overlaps the fixed block, and which is generally not associated with a fixed block size (that is, from common parents, only the body tag). filter: blur blurs only the fixed block, and not what is behind it (text, images, etc.)

Have reviewed everything I could find, but never found an answer. That is, writing to the customer that this is impossible?
March 23rd 20 at 19:33
1 answer
March 23rd 20 at 19:35
So?
I need the blur was only for transparent popup window, not the whole page - Gerald_Kris commented on March 23rd 20 at 19:38
@Gerald_Kris, - catharine77 commented on March 23rd 20 at 19:41

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