I am sorry you had to suffer through a Matt Osborne YouTube video just to learn about diffusion filters. These guys are mostly just recycling stuff from old Popular Photography magazine articles, which is where I learned about diffusion filters 50 years ago. A diffusion filter is a good choice when you want an image to look like you are using a diffusion filter. A diffusion filter doesn't really look like using an older lens with less resolution, but I guess it might fool some people. I thought you recently bought an old Cooke Amotal lens that wasn't super sharp that you liked. I'd recommend you use that rather than a diffusion filter if you don't like the look of your super sharp lenses. Of course, if you want an image to look like you are using a diffusion filter, only using a diffusion filter will do. And I don't know what this Black Mist stuff is all about. It's a Tiffen. Marketing I guess. I am not sure what Black Mist even means. It sounds like a villain from a Spiderman movie or something.
Of course that advice only applies if you are shooting film. If you are shooting digital, skip the diffusion filter and add the diffusion effect using software. That way you can add just the right amount for the image rather than having to compromise with the fixed amount from the filter. Also, if you later decide you do don't like the diffusion effect, you can undo it. If you shoot with a diffusion filter, you are stuck with it. Here is a YouTube video on how to achieve the Black Mist effect in PS/LR. Unfortunately, the video is not as awesome as it would be if it were by Matt Osborne, but it is passable nonetheless. Basically, it is about adding blur in PS.
https://www.youtube.com/watch?v=45UjziaLsyA
If all of that sounds too complicated, just bump your sharpness and contrast sliders down to take the edge off your super sharp lenses.
If you like shooting jpgs and don't won't to be bothered with editing your images, I guess a diffusion filter is your only choice for making images that look like you are using a diffusion filter.