Ah - now I understand what you are trying to do...

Visual FX 070 is a black mask which works with the same keystone points... put it on the layer above the content and use keystone points to adjust.

Do you not also need to keystone the image too??

for reference:
Visual FX 195-202 - Crop - allows you to crop the content without changing the scale - this might be another way to do it...

Visual FX 001 gives an infinite black border around the outside of the content no matter what the scale - so you wont see layers beneeth

Simon