Ok, here is the exact breakdown of my issue:
-Create a composite shot
-Import my background plate (still pic)
-Import my green screened actor
-Key out screen with Color Diff, Spill Remove
Looks good. Now I want to blur my background plate to make it "out of focus"
-Add "blur" filter to background plate (radious: 10px)
And Viola! My video disappears! My actor has vanished. The footage is still in the composite shot, but cannot be seen in the preview screen (and doesn't render out either)
edit: I updated my drivers to no avail.