I had Canon F1 cameras--removable pentaprism, interchangeable screens. There was always dust. Dust on the top of the screen, dust on the bottom of the screen, dust on the bottom of the pentaprism. Also a Horseman 4x5 view camera and a variety of medium format cameras, all of them with everything interchangeable and lots of places for dust to enter on every side.
There was the possibility of a bit of grit in the plush lips of the film cartridge--that gave you a solid line across every image of that roll that was an absolute devil to deal with in printing. If you "rolled your own" film cartridges, the possibility increased enormously.
If the environment was dirty enough, you could get dust on the film, which was also a devil to retouch on the image because it printed black--you had to take the point of an X-Acto knife and literally chip away the emulsion of each and every dust spot of each and every print you ever made from that negative. This didn't happen much with roll film, but it's a constant evil with sheet film. I’m not even going to get into the process of cleaning and loading sheet film holders while keeping out dust—that’s two or three paragraphs right there.
Then there was the possibility of "air bells" on the film as you processed it. A lot of people pre-soaked negatives in water to dispel air bells, but that could cause a problem with high-dilution or very fast developers. Otherwise, you used fancy hand techniques to knock off air bells and ensure even development. Guys developing film waving the tanks in fancy patterns often looked like Merlin doing an incantation.
Then you had to dry the film, and you faced the possibility of a bit of grit on your squeegee or a hangnail on your finger gouging scratches in the soft emulsion.
Then you had to deal with dust on the negative when you printed it--EVERY time you printed it. The first step was to keep the darkroom absolutely spotless so that there was as little dust as possible in the room. You humidified, vacuumed, wiped, washed--enough to intimidate even Mary Poppins. Some people even resorted to darkrooms with positive atmospheric pressure systems.
Then you brushed and wiped and washed (using a highly toxic degreasing chemical) and blew and used an electron gun to >zap< your negative or a radioactive brush to *irradiate* your negative to get the dust off...every time you printed it. If you held the negative under the enlarger light just right, you could see each dust mote and dab it off. Oh, but don’t forget first to tap the negative against the enlarger to remove any static charge.
And after all that, there were still the ghostly white images of dust on the print. So you got a couple of bottles of Spotone in different tones and a #0000 brush (with half the hairs plucked out). You carefully mixed the Spotone by eye on a plastic palette to match the tone of the print, but you didn't use it directly.
First you let the mixed Spotone dry into an inkspot on the palette. Then you wet the brush with your own saliva (because it adhered better) and deftly dappled the color over the dust spot in such a way as to duplicate the texture of the film grain.
And you did that with each dust spot on each print from that negative.
Digital images and Photoshop…"Dust? I laugh at thee! Ha-ha!"