Aha, good sleuthing! Doing some Google searching for Qt5 black icons then came up with this comment on the Debian mailing lists from @rikmills , one of the Kubuntu maintainers: Re: Black folder icons in Dolphin
The reason is that the kf6-breeze-icon-theme which replaced the kf5 version has some changes which are not supported by the Qt5/KF5 kiconthemes library. You are also likely to see black folder icons in the file picker of any Qt5/KF5 app when you enlarge the icon view enough to see coloured folders.
Some solutions might be:
- downgrade to the KF5 breeze icon theme package and put that on hold with apt until Qt6/KF6 apps are ready. - as some have done, copy locally the old icon theme so that takes precedence. - persuade devs to revert the offending upstream change in the debian packaging, again until no longer required.
Hope that helps.