When screen sharing in Zoom, sometimes black boxes can appear to the viewers.
In order to stop this from happening, you will need to adjust your screensharing settings in the Zoom app:
Begin by opening your Zoom app.
Click on your profile picture
Select SETTINGS when the drop-down menu appears
Select SHARE SCREEN and the ADVANCED button
Under the ADVANCED options, make sure that the SCREEN CAPTURE MODE is set to AUTO or ADVANCED SHARE WITH WINDOW FILTERING.
If the boxes still appear to your students when this setting is set to auto, switch it to the ADVANCED SHARE WITH WINDOW FILTERING.