Have you tried just opening a full-screen menu like the map or the graphics settings screen? I am also having this issue where the screen gets fully enveloped in black on an unpredictable basis and have found that opening a full screen menu (or, weirdly one time, getting off my horse?) fixes it. I haven’t yet had to quit the entire game to fix it.
https://us.forums.blizzard.com/en/d4/t/black-screen-bug/55242

