Why does my screen go blank when I cover the cam on my iPhone on the Skype app?

I have an iPhone 5 and when I'm on Skype - even when I'm not in a call and I cover the camera my screen goes blank. It also does this when I'm on the home screen and signed into Skype but not when I log off - I don't know whether this always happens and I've just never noticed it or whether something is up? X

All cameras go blank when the lens is completely blocked.