Good trick: you can actually track each corner on individual nulls and then link the corner pins to their respective nulls with a pick-whip expression.

Rack focuses are one of the tricks, like adding noise or shake or fucking lens flares, that can be a big help in tying a composited element into the scene. If you match the focus right, anyway.

I wouldn't use a blue screen for that, but you don't want it blank, because you'll need to be able to see the corners when it's out of focus. I'd probably use one of those brightly colored post-it sticker notes, divide it into four little squares a centimeter or two across, and paste those on the inside corners of the screen.

Track, get your corner pin data.

Then go back over them with the color of the screen, before adding anything to it. So it's like they were never there. You should be able to to just make a solid layer the color of that region of the screen for each corner and use your existing tracks to stick them on.

Then add screen, enjoy beer.

Says the writer of apocalyptic show tunes? 

It's back