The first thing I learned from a master re-toucher was, get good red sable brushes. Then get a plastic water color palette, using an eye dropper or pipette, put about 1 ml of the approriate spot tone fluid (probably the neutral) into one of the "dimples". let it dry out like water color cakes.
First trick. Wet the brush, mix a bit of the grey, starting WAY LIGHTER than you think, and then, before touching the print, the brush has to be, what you'll at first think is almost dry, void of dye. Then you can begin. The strokes must be feather light. I had always learned to use a stippling technique, but my bud Manfred does more of a light short stroke. Patience is the key and use only a lightly moist brush. Larger spots should be filled in starting at the perimeter and work your way to the middle. Here the key is only touching the spot, and not the surrounding field (especially in the case of a sky) which would then cause a dark ring to be created.
For small black spots caused my dust in the film holder, you can carefully "flick" it off using the sharp point of an exacto knife, then spot as usual. For larger areas, you can use bleach to lighten and then re-darken using spot tone and re-fix using a q-tip or larger cotton swab. I'm not at this level yet, but I've watch in amazement as Manfred (a real old time pro) demonstrated his talents on some of my prints. Just for kicks, I watched him eliminate the jet stream from a passanger jet that ran through an otherwise cloudless sky. And you could NOT tell it was even there when he got done!
As a side bar, Manfred use to retouch my color transparencies at work when necessary. That too was a treat to observe. It's too bad that a man with his talents is not really needed anymore, professionally speaking of course.