A black screen can appear if a user’s camera is broken or installed incorrectly. Test your camera with another camera application like https://apprtc.appspot.com (click join) or Skype or restart the browser or refresh the page.
Another possibility is that the incorrect device was selected when allowing the camera.
The third possibility is that the recipient's (attendee) connection speed might be slower than necessary. A slow connection speed can prevent them from connecting and camera from loading. If this is happening to certain attendees, we recommend they do a speed test.