Mr. Ball has twice experienced his monitors going black intermittently. This seems to be related to an interaction between the output of the Mac and the AV equipment. It has never affected the TV in the room, but both other monitors and the projector are black. It might be related to specific monitors, swapping monitors fixed the problem the first time, but didn’t the second time.
The second time, changing the resolution from 1080p to 720p fixed the problem. It stayed fixed after changing the resolution back to 1080p. You will probably have to hold down the option key when you click on Resolution -> Scaled to show all of the options (What the hell, Apple?). I am not at all confident in this solution. I messed around a lot before this worked, and I don’t know if it was this, or this in some combination with what I did before. I also don’t know why this happens when it happens, so I don’t have a way to test it.
Third time I tried replacing the USB-C to HDMI converter with 3 others and that had no effect. Eventually after some restarts and changing to various other resolutions, it worked. But changing back to Mr. Ball’s username broke it again. His display settings had the monitor that wasn’t working at 30Hz instead of 60. Changing that fixed it. Would that have fixed it right at the start?