If your video feed from Facecam is showing up as a black screen, you can attempt to fix it by attempting these troubleshooting steps.
Facecam Is In Use By Another Program Or Application
Facecam can only be accessed by one program at a time, so make sure that you double-check any programs that are open and that are not accessing your video input feed.
Some programs, such as Discord, Microsoft Teams, and Zoom, hide in the taskbar when closed, so make sure they are fully exited.
Disconnect and reconnect Facecam via the USB cable to reset the video input feed.
Facecam’s Lens Cover Has Not Been Removed.
Make sure the lens cover has been removed before use.
Checking Settings in Camera Hub
Check Camera Hub settings to make sure the image isn't darkened by a specific setting. Specifically, Shutter Speed and Compensation can greatly affect how dark your image is.