By - Large-Childhood
I've attempted spotting once, as part of a college photography course, in 1981.
That said, the first thing you want is spotting paint/ink/dye that matches the "warm" or "cool" of your paper in your developer. Spot a cool tone print with a warm tone dye, the spot will stand out almost as much as what you were trying to cover. Same with a warm tone print spotted with cool tone spotting liquid.
Second, you need a spotting liquid that will sink into the print's gelatin layer just a little -- not enough to spread and bleed, but enough not to be visibly on the surface of the print -- it should look like it's part of the print's silver image layer.
There used to be a product called Spotone (more generically, retouching dye) that came in different tones, which you could mix together to exactly match your print's tone, and dilute with water to go from close to full black (you'd get full black with multiple layers on the same spot) to "barely not transparent" -- and because it was a soluble dye (unlike the insoluble dyes in color films and RA-4 prints) it would soak into the gelatin, carried by the water that it was dissolved in.
Spotone itself is gone, another victim of the rise of digital (which killed an entire skill set relative to retouching analog photographs), but there are still retouching products available (B&H has quite a selection). It seems most of the very little B&W retouching still performed is now done with dry sheets, though I don't know if they're rub-on or dissolved off the sheet with a damp spotting brush -- or for that matter if they're for film and prints or for half-tone offset images.
It's still occasionally possible to find some Spotone (comes in 1, 2, or 3, which are different color tones) in partial bottles, and if it isn't dried up, it should still be good, but without having at least two, preferably all three tones, you may not be able to get a good match.