How to make second monitor go black?
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.