I want to bump this thread, as I am having the exact same problem as OP. I am trying to play mp4 videos from a source I have been using for a couple years with no issues, but now only see a black screen with correct audio. I can't say for sure if the problem started after an OS update.
When I play with VLC, it plays fine. When I Airplay to my 4th generation Apple TV from Quicktime, it plays fine on the TV. If I play the little thumbnail preview in Finder, it works fine. I have a touchbar MacBook Pro, and the scrubbing bar in the touchbar displays the frames just fine. Now I just noticed when I turned Airplay off in the middle of playback, it continued on my computer just fine, but when I skipped back to the beginning it stopped playing video again.
The videos are already encoded in h.264, so not sure what I can do there. Any ideas?