Apparently this is also known, from the 6.5 release notes:
We’re also following up on an intentional change to the blur effect that unintentionally made it uglier in some cases.
Reddit thread about it (50 upvotes, was posted 2 hours after this)
https://old.reddit.com/r/kde/comments/1of6gwa/why_does_the_blur_effect_look_so_weird_colors_are/
bugzilla link https://bugs.kde.org/show_bug.cgi?id=510818