Ok, Rgba(0,0,0, 1) or whatever color with an alpha value of 1 is definitively a bad idea !

It could be noticed a little depending of the screen color and the viewing angle for a LCD screen.

Moreover, my co-worker who is the infographic artist told me that it could be far more noticeable on CRT screen with a particular contrast.

I will have to dig and I'll try with the QDesktopWidget