It's not as simple as it looks. Thankfully - the VMF of the map where the Alyx rescue scene happens is included with the SDK.
I worked it out by looking in there. Look for an entity called "blackout" in that map, and you should be able to work it out from there.
You have to use an invisible model, which is a bit of a pain because you can't see it in the 3D view -- you can only get to it by finding another entity which has an output or input relating to it, and selecting it that way.