You do so with an http://geck.gamesas.com/index.php/ImageSpace_Modifiers. The wiki page explains what all the settings do. In the geck, I think under miscelanious (its not open right now) is the ISM tab. Click that, then say New. Then you apply it via the http://geck.gamesas.com/index.php/Imod command in a script. You want to go from nothing to a 255 black alpha. That will make it fade over x second to black.