you can customite this script by editing the first 2 commands :
Define the classes you want to exclude
excluded_classes=("plasmashell" "Thorium-browser" "kwin" "")
Define the window types you want to include to not be blurred :: you can remove "_NET_WM_WINDOW_TYPE_MENU" from this list if you also want blurring behind menus (context menus etc...)
included_types=("_NET_WM_WINDOW_TYPE_DOCK" "_NET_WM_WINDOW_TYPE_DESKTOP" "_NET_WM_WINDOW_TYPE_SPLASH" "_NET_WM_WINDOW_TYPE_DND" "_NET_WM_WINDOW_TYPE_MENU" )