Yeah, it’s EBSynth to generate the black outfit frames and then some AE masking to comp just the edited outfit in on top of the original footage, so it’s fairly accessible but time consuming. Scenes on Kef Bir or Exegol, or with lots of movement in general, would probably be a lot trickier though.

Though I enjoy the look, I’m not sure it would really add much to the movie besides being a ROTJ/ROTS callback (and, as with Luke and Anakin in those movies, playing on the audience’s fear of the main character’s changing relationship with the dark side).