Actually, back in the 70's (I think), Fuji came out with their first 35mm SLR (film camera) ST-701. There were three prominent advances in that camera. There was a "silicon blue-cell" metering system which was faster responding than CDS cell systems. There was a silver coated penta-prism which made the view finder much brighter. Lastly, they used black "flocking" inside the mirror-box to reduce light bouncing around and fogging the film. All three advances worked. The only one that any other cameras picked up on was the silicon "blue-cell" technology, and that was only used in a couple of cameras outside the Fuji range (I think one camera was a Chinon). The black "flocking" was obviously superior to the matte black paint used by the other cameras. You could see it clearly. It looked seriously black where all the other cameras looked dark grey. The ST-701 was a relatively inexpensive camera, but was similarly high quality to their X-T30 and X-T4 lines today, so the "black flocking" could not have been expensive. Yet no other brands followed that lead.
I doubt if you will see this picked up in todays cameras.