To prevent other people from possibly seeing black boxes when you share screen, this setting needs to be changed:
1. After starting screen share locate the Screen Sharing tab at the top of your screen.
Hoover your cursor over any part of the screen sharing tab to see the Meeting Controls
2. Select More
3. Then select Hide Floating Meeting Controls. This causes all floating controls to disappear.
NB: You will not be able to access the Screen Sharing tab or Meeting Control in this mode!!
4. Press Escape [Esc] or Ctrl+Alt+Shift+H to reactivate the Screen Sharing tab and Meeting Controls.
For more information see: Sharing your screen or desktop on Zoom