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
Best Answer
See my question as my answer. I wanted to post as answer in the linked thread but I was unable but I wanted to give back to a site that has helped me in the past