Maybe someone could find this useful.
If you have a better solution, PLEASE let me know!!!
I figured out a way that works, but i don't know, if it is the best/correct way.
It works with Backbuffercopies:
The BackbufferCopy-CopyMode needs to be "Viewport"
The shader is in the BlurLayer, and these Polygon2D are covering the whole viewport.
The result is something like this:
Here is the shader-code:
COLOR = textureLod(SCREEN_TEXTURE, SCREEN_UV, 1.0);