The most common reason why you see a blue screen is because a device (typically a TV card) other than your webcam is being used by Yahoo! Messenger. If this is the case, you can fix this by changing your camera source. Here's how:

1. Select " Yahoo! Messenger."
2. Select "My Webcam."
3. Select " Yahoo! Messenger."
4. Select "Preferences."
5. Click "Webcam."
6. Click "Camera Source."
7. Click the drop-down menu and select the desired camera.
8. Click "OK."