@RobinK I tired it with igormats suggestions. With most of my Shaders it possible to create a Fullscreen Effect (on Mobiles too). Most of the blur Shaders that I found so far are to slow for mobiles.
I kept the solution with the additonal semi transparent Layer. For a Messageboxs Background it’s okay.
There are some other solutions like 1. creating all needed blurred SpriteFrames at initialisation and save them on device. 2. faking Real-time Blur. There are some LibGDX / Java solutions. In my case it would be to much effort just for a msg background!