My Sony Vaio’s built in Motion Eye camera is showing black. It only shows light, but in green. Here is a YouTube video to demonstrate it: http://www.youtube.com/watch?v=zcLNRwgexJk
Are you using Vista? It appears there's a known problem with Vista and the Sony Vaio cam not starting properly - see the fix here:
Not sure if it's also an issue with Windows 7. Hope the link helps!
Did you check out the link above? Did it help?