Why use black video?

I had a look at the tutorial, the plugin is not using a normal scale that can be applied to the original clip, the plugin is creating new pixels, the original clip's resolution is FHD, and there is no way to double its resolution directly.

So, you need the black video because you need a 4K placeholder to be able to hold the newly created pixels by the plugin. That's why I believe you can use any 4K footage instead of the black video, apply the plugin to it, and you will have the same result. Of course using a black video would be better, as you can change its duration easily to match the FHD clip.

You will have to ask BorisFX as why they have designed it to work that way.

@R Neil Haugen btw it's a 2 layer stack.

I find the tutorial kind of messy

(2160p with non-square pixels in one image, as said SD to UHD is not going to give a good result).

It looks like they are using a customized version of the Track Matte effect on the black "slug" clip on V2. That's why. The steps are nearly identical to normal Track Matte usage.

With of course, the output on V1 it also appears.