Photoshop is very popular for retouch pictures taken by photographers. But how did photographers touch up photographs before the invention of Adobe Photoshop?
A photographer’s skill to retouch film and spot a print was a make or break proposition. Photographers had lots of tricks up their sleeves and successful ones used all of them. The portrait photographer made heavy use of these skills. They retouched negatives to remove blemishes and soften wrinkles and the like. Likely a retouch artist was on the payroll or the work was sent out. There was a thriving cottage industry and these artists also colorized black & white prints using transparent and opaque oil paints or water colors.
We often used portrait lenses that were intentionally soft or we mounted softening filters. This minimized the need to retouch. We favored large format cameras because the larger negatives lent themselves to be retouched. We mainly retouched using a pencil. We blended wrinkles and birthmarks and removed pimples. We worked both sides of the negative. We had colored pencils to use on color negatives. We had water color dyes, and we applied these with a fine pointed brush.
Portrait film likely had a “tooth”, a roughened surface that permitted the use of pencil. We applied a “toothed” lacquer to films without the “tooth”. We made masks: These are duplicates of the negative. We made them by contact printing the negative onto another piece of film. We sandwiched the mask to the negative. Between the mask and the negative was a spacer. This softened the focus of the mask making an un-sharp mask that merged the mask and the negative perfectly. We made a negative mask to increase contrast. We made a positive mask to decrease contrast.
Photo prints were always dotted with white spots. These are undesirable blemishes caused when dust settled on the negative during printing. We did everything possible to prevent. We made the darkroom a clean room. We neutralized the electrical charge of the negative, even using a radioactive negative brush. We use glassless negative carriers. Despite our best efforts, we got white specks that had to be spotted. We used pencil and dye.
Bottom line: You got it easy compared to what we went through.
For removing spots there were fine brushes with paints designed to match the tones in black and white prints (I still have a set, 'cause I'm a geezer). For smoothing skin you could use airbrushing. For manipulating contrast in specific areas you could use masks (physical masks, cut out of thick paper) during the print exposure, so some areas got more light and others less. For dropping in part of two or more negatives onto one print, again masking. Theoretically you could also paint on -- or scrape away at -- the negative itself, I'm not sure how much of that was done. For prints that were going to be mass produced (so no one is really looking at the original print) they might have even made a collage, again theoretically.
Wikipedia gets into this in the article on Photo manipulation.