Right now applets on the desktop have transparent backgrounds, but they are not blurred like the transparent backgrounds of the panel and panel applets. They should be, both for reasons of consistency as well as usability: visually busy wallpapers can cause the content of a translucent-but-not-blurred applet to become more difficult to distinguish, especially with Plasma themes featuring very high levels of transparency.
I talked about this to Marco. I'd also like to blur them, but it's not as easy since they are not considered windows by kwin.
Yeah, I had a feeling there was some technical barrier (because otherwise it would have already been done!).
Fixed by Marco Martin with https://invent.kde.org/frameworks/plasma-framework/-/commit/532817c00455ef7ddb400529b9c102f142db508b and https://invent.kde.org/plasma/plasma-workspace/-/commit/e1980bc024ab4df234dd5104a9100f7cd95b43bc in Plasma 5.23!