I am not sure how to begin with this explination. I guess I would need to know more about the scene. I would probably use a chromakey Green Screen or Blue Screen (lighting in the key to make that work) then add in a black .jepg file... Let me show you like this. I have some chroma-key green footage

In the scene above, I have taken the greens screen chroma-key and added a back background. The project looks like this...

I cut the video only to show you that initially it was a green screen, I then added the chroma-key video effect to the second part of the cut video while also faded that in one second so that you can see a video fade in using chroma-key (Green).

I hope that makes sense.