Blur screen in Android service

I am creating an app with a service and I hope it can blur the screen at any time, even when the app is in the background.

I have created a service that, when an alert is called, creates a transparent view at the top of the window. This is where I got stuck. It seems that what's on the screen (no root) isn't enough anyway to create the blur effect and put it on this top view.

Is there anyway I can make this work? Is there a way to capture the current screen outside of my application? Or could I do something with a transparent view to do something under the blurred view, like looking through a blurred window?