Turning the finger black, without swelling, irritation, or signs that the skin itself is reacting can often be traced to cosmetics. Many of the cosmetics with pigments in them like rouge, zinc oxide, or others, can do to a finger coated with that pigment, the same thing that a polishing compound does to a buff. The buff turns black, even with white or yellow compounds. The black consists of very fine metal particles, removed from the jewelry by the compound, and now coloring the buff. If the cosmetic material is able to do this (remove metal just as polishing compounds do) then the result will be that same black color on the finger, as happens to a buff. This is not to be confused with something turning the finger green, or the ring itself becoming stained or oxidized, or instead of the finger turning just black, a finger showing signs if inflammation, redness, etc. For the cosmetic produced problem, the client should remove her rings while she is applying makeup, and then wash her hands before putting the rings on again. Rings made of softer alloys may do this more than rings made of harder metal, just as some alloys are quicker and easier to polish.
Hope that is useful.