I tried to reply to this thread - MacBook Pro detects external monitor but shows black screen.
Unfortunately I don't have reputation to do so...
If you are having troubles with a black display it could be that your Mac has onboard graphics and a dedicated graphics card.
The full issue I had was a 2018 MacBook 15inch using two hdmi to usb-c connectors, only one display would ever have an image, the other was black.
As soon as I turned off Automatic Graphics switching (Apple Icon > System Preferences > Energy Saver > Tick Box for Automatic Graphics Switching) the problem immediately ceased.
Hope this may help some others with the same issue as me.
Edit: Ensure you do this for the relevant mode of use. You can change this setting for both using battery and being connected to power