Chosen Solution
blue background is from Youtube RGB test - when I move mouse to see title of video - is even more visible - but I suspect it is because transparency - but why when plain blue colour? I is only barely visible on red background - other colours - no. And it is highly visible something when Safari empty tab opened - when dark grey background Is it normal?
I’m sure what you are seeing is the persistence of the image not the OS’s Transparency selection.
