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:
Select " Yahoo! Messenger."
Select "My Webcam."
Select " Yahoo! Messenger."
Select "Preferences."
Click "Webcam."
Click "Camera Source."
Click the drop-down menu and select the desired camera.