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. You need to change your camera source.

To change your camera source:

1. Click the "Messenger" menu in Yahoo! Messenger and select "Start My Webcam."
2. Click the "Messenger" menu in Yahoo! Messenger and select "Preferences."
3. Click the "Webcam" category.
4. Click the "Camera Source" button.
5. Click the pull-down menu and select the desired camera.
6. Click the "OK" button.