This is a tough one. I have an idea, but I'm not sure what it'll do. You might want to wait for Killer-Duck before you try my idea.
1. Create a thin brush and make it func_wall. In it properties set:
A) rendermode = Texture
B) FXamount = 0
1.. That's your window.
2. Create a HUGE brush that fills up the whole room and one side of it is touching the window. Make it func_illusionary, and texture it solid black. This means you can be in the black func_illusionary, and people from outside see only the black. This should work fine on the window side, but the rear end will be black and weird.
This is a bug/feature in the Half-life engine, brush-entities don't block light.
I'm not sure of this, but if you're using latest version of Zoner you could go into the properties of the brush-entity and set "Light Flags" to "Opaque (blocks light)", not sure if it will work though...