Black Screen after using CMD+TAB
This is currently a known issue. Using windowed mode will work around the issue in the mean time. In the Blizzard App game settings menu (Options > Game Settings), check the Additional command line arguments and enter in: -DisplayMode=0
If this does occur and the screen is black, restarting the mac isn’t necessary, press CMD+M to set it to windowed mode and bring the Heroes of the Storm client back.