Others have asked how to remove the still background from a video. I'm not asking for that because I recognize it is problematic
Here's an easier challenge:
For many "talking head" videos, it is sufficient to take a still without the person, video the person and then provide the still as the background for removal. (Replacement would then be trivial with even the lame chromakey algorithms that abound since the background would be flat black or easily set to a constant color, like green screen green.)
In terms of
blend it might be accomplished with a series of operations that divide the video by the constant image (producing a near-constant ratio for the areas matching the background), and then setting that constant to some color. Although this, quite deliberately, isn't an
ffmpeg question since I suspect this is a function so widely useful that it should have been packaged.
I would be surprised if there weren't programs that do this.