Ubuntu 16.04 windows or screen go dark when busy or not responding
Go to System -> Preferences -> CompizConfig Settings Manager -> Effects -> Fading Windows and then just to option for Dim Unresponsive Window and you can set the option as per your requirement.
And Ive tried: Unity Tweak Tool
This doesnt seem to work in 16.04, and Im getting really tired of not being able to see much half of the time, even with core2 2.33 ghz and 8g of ram and a good ATI graphics card. It seems to gray out nearly anytime the system is slightly busy