If you see a black screen when broadcasting your webcam:
There might not be enough light in the room. If so, illuminate the room appropriately.
Your camera might not be functioning properly. Try your camera with the manufacturer's software.
Yahoo! Messenger might have the wrong camera selected. To change your camera source:
If you see a black screen when viewing a friend's webcam:
Check the status message at the bottom of your viewer window. If it says "Stopped," the person broadcasting has stopped broadcasting images. Ask the person to resume broadcasting and try viewing again.
If the person is in fact broadcasting and you still see a black screen in your viewer, ask them to check their camera and take the webcam compatibility test.