This feature actually did not make it into 4.8, not sure about 4.9 but guess it’s not in there, too.
See this thread: https://forums.unrealengine.com/showthread.php?70143-So-Blurred-glass-material-is-impossible-in-Unreal-Engine-4
RyanB seems to be the guy behind this Material(I contacted him to better understand it., why it is not in the engine etc.). You can create a Custom material node and copy/paste the HLSL shader code (from the thread above) to create the “SpiralBlur” Material.
I tried it and we used it for the blurred glass in our biotic project: https://forums.unrealengine.com/showthread.php?83986-RealtimeVR-Biotic-%E2%80%93-Interactive-laboratory
First of all, it is heavy on performance! It looks ok with lower settings but it’s hard to use for realtime use cases with nice looking results. In the thread, there are mentions of other methods that maybe fit your use case better.
Anyway, good luck with it 