Kwin, currently, has no setting for allow the background of transparent windows to be blurred, despite this functionality already existing. The panels, popups, Application Launcher menu, and Krunner are already blurred, but it is impossible to blur windows like Konsole, despite transparency being an option. I am aware a blur option already exists in Desktop Effects, but it only works for the aforementioned.
Blur is an opt-in feature by the windows. A window needs to pass the region it wants to have blurred through a windowing system specific API. There is an API call in KWindowSystem for it. E.g. konsole needs to implement this, which is bug #198175. From kwin side we cannot just blur every translucent window as e.g. GTK+ uses translucency to add shadows. This would look really bad and destroy performance.