Depends on your paper and developer as well as toner. For most selenium toned prints, some ratio of brown and neutral black Marshall's (not blue-black) will work. But I alway keep on hand Olive Tone too - a tiny bit of it shifts things yellowish. It not only takes practice to get it right, especially in high key areas, but a good color eye. A good spotting brush is important. These are hard to find. Camera store versions are miserable. And you want a scrap print nearby, or use of an unexposed
print border to test the exact effect. You want to slowly build up density without a minimum of dye on the brush. Incidentally, Marshall and Spotone are the same thing; Marshall's bought Spotone, thank goodness. Do not get the colorants for color print retouching, but those specifically recommended for black and white. Otherwise, the long term results in terms of either fading or darkening might be unpredictable. Best to stick with something which has a proven track record. I happen to have
a very nice retouching station. It has a big black laminated surface tilted to about 30 degrees, with an adjustable desk lamp above, containing two CRI 98 5000K true color matching tubes (not the kind of thing you can find at a hardware store, but
equivalent to what Just Normlicht uses in their high-quality lightboxes), plus my ole black leather plushy office chair - itself certainly proven to be comfortable!