If you disable windows transparency effects then all blur will be set to solid in the OS.
The taskbar will drop to non blur mode, but this would be a very bad idea for a menu which appears over other content containing text. Would often be unusable.
It seems like, if you were able to adjust the transparency like the taskbar, say 75-80%, it would provide enough blur and be usable.
Also, with my menu style, all the Tab Groups are solid, so it wouldn't really matter.
If this feature is something you have chosen not to provide, based on your reply, I can respect that.