Ok, so this is weird.
Turns out someone had the same idea as me, only 3 years earlier in 2001 and wrote a utility and called it Blackout (!), it's freeware and it might actually work over dual screens and stuff!
You can download it from here:
http://www.softpedia.com/get/Others/...Blackout.shtml
Let me know how you get on, I've only tested it on my laptop so far.
My only niggle with it is the flashing text every now and again. Ah, who am I to complain!
