Blur shader is anarchist and don't collaborate

[gas]

Ok, my attempt was to replicate an AERO interface, using the blur shader to create the glass effect.
The thing is made by blurring an area and then draw on an upper layer the semi-transparent window.

This.

damned_shaders.jpg (6.28 KiB) Viewed 555 times

The blur effect is quite requited to make the text readable on top of the "glass", as for GLASSMORPHISM paradigma.
And yes, if something move behind the window, it should blur dynamically (figure out a change in scene or a character moving left right behind that glass...).

My attempts so far leaded to... nothing.
The first logic was to append a new layer properly scaled and apply a blur shader to it (the blur shader shipped within renpy, to be precise). Nothing happened.
In the second attempt I've tried to apply the effect to a "fake" png image made of 100% transparent pixels, scaled accordingly. Nothing (or probably the blur is applied to those pixel ALONE, not the entire area below of it).

I'm a bit confused right now on the actual implementation.
Can someone tell/verify/validate my concepts?