I have a second display connected to my iMac and I want to make it go black some times, for example if I am watching a movie on the first monitor.
Apparently, there has been an app called Screen Commander but it is not available any more.
I run macOS High Sierra on an iMac from 2010 and have a 24" Apple Cinema Display attached via Mini Display Port, of course.
I can not imagine why there is no tool which does this! I know I can set up different spaces for the monitors but this is not what I want.