If iGlasses only shows black on macOS 10.15 or above, you should check to make sure camera permissions were properly granted.
To check and fix these permissions at any time:
Open the Security & Privacy section of the System Preferences app.
Pick the Privacy tab.
Look for the the Camera section in the left-hand sidebar.
Click the orange padlock in the bottom-left to make changes, and enter your Mac's admin password when prompted.
Check the checkbox for iGlasses if it's not already checked. It may be there twice. Check both.
If it was already checked, uncheck it and re-check.
Then, close all video apps and try again.