Hi mildew, welcome to SAF
I never heard about something like this... sounds like some virus action
Tried to reproduce this on my desktop, but no joy... black screens are normally found when you boot to safe mode or if you have the "high contrast" settings enabled on <Shift+Alt+PrintScreen>
Are you using some reliable antivirus ?
Are you able to run HijackThis on her system ?
If yes, please open a new topic on "Malicious codes" forum and post the log there.
If you need some help on dealing with HJT, please take a look
hereChris