How to make second monitor go black? 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.
If you use VLC for playback, it has two options to black the other screen during fullscreen playback.
Use native fullscreen mode,  or
Black screens in Fullscreen mode  

They are mutually exclusive, enabling native fullscreen disables the other.
Native requires a restart of the app, 'black screens' can be switched on &amp; off at will.
I haven't tested, but one may work better than the other depending on whether you have Spaces set to move in pairs or separately, in Mission Control prefs.