sadly it's not currently possible, though efforts are being made to make it possible ATM. In the meantime, I think Ubuntu MATE with compiz can blur behind windows :)

To expand on this answer, KDE plasma now blurs beautifully behind windows and has a pretty cool theming system too :

Here's my desktop ! you can check out how I achieved it here : https://www.reddit.com/r/unixporn/comments/8idl9p/plasma_shameless_macos_clone/