You will have to forgive me. I work in Film Editing, not VFX. Also, as I mentioned, none of these techniques are used anymore. CGI was already around when I started in the business. Digital Intermediates started replacing cut negative and photochemical Opticals about 6 years after I started.
Hi-Con film is a type of B/W where there is no Gray. Stuff comes out either as completely Black or as completely White. They use this film when shooting title cards as well.
Blue was used because of the three color layers (Red, Blue, and Green) on film it had the finest grain structure. Green is used for CGI because it has the highest luminance and can be the easiest for computers to see.
Have you ever seen an Anaglyph 3D image or movie? If you look at the raw image, there is a blue and/or red off set. When you look at it through glasses where one eye is blue and the other is red, The eye with the blue lens does not see the blue offset. The other does. And same for the red offset in the other eye. Your brain then fuses them together.
It is basically the same idea here. If you use color filters when running a Blue Screen element through the optical onto Hi-Con film, you can get it so the camera doesn’t see the Blue. The light shines through everything that is bright blue and hits the negative turning the silver emulsion black. Anything that was not blue is masked. No light hits the negative and it stays white. Because it is Hi-Con film, anything gets even a little bit of light turns to pure black.
Now that you have a hold out matte where the actors are black and the blue screen is white, that can be run through an optical printer to make the reverse matte where the actors are white and the blue screen is black.
The one where the actors are black will be used when running their element through the optical printer. The one there the background is black will be used when running the death star element. Same would go for the the Tie Fighter element (with its own matte). You use the mattes to prevent double imaging. Otherwise you would see a little bit of the background superimposed over the foreground elements.
Blue screen and B/W mattes are still used in CGI. But created in a totally different way.