When I select "Inactive Blur" plugin in Wallpaper settings the pictures/wallpapers below all disappear. I tried to re-install "inactive blur" plugin (also tried the ubuntu version), all without success.
I think it was dropped for good since broken again in KDE... See:
So, basically, if your system was installed in the last couple of months, you don't get the plug-in at all; if it is an older system, the plug-in is not working anymore (see also issue list in the github linked in my next post).