We Chinese call it "black money", too. But it sounds too harsh and directly pointing to crime when talking, esp. publicly. So the color is changed to gray meani

We Chinese call it "black money", too. But it sounds too harsh and directly pointing to crime when talking, esp. publicly. So the color is changed to gray meaning color between clear and black so that no one "overreacts" (when "black money" is normal, everywhere possible and most people deal with it do not end up in jail or are not being investigated"). Just a way people accept the reality.