How to key a black and white transition video in AE?

There are nice transition videos on Stock. I figured out how to use a colored one by simply getting rid of the black with keying and then it overlays to make a nice transition from scene A to B. See below.

But they have others that are black and white. See below.

I could key out the black revealing scene A but I'd like the white movement in the transition reveal scene B. I'm sure there's a way to do it but can't figure it out. Can anyone help explain how to do this?

Thanks.