Sherman, set the WayBack machine to 1975, Mr Buscemi's chemestry class......
As I recall, you can take bromine, coat copper with it (and frets are essentially copper), and its active enough, being a halide, that its reaction with the copper would turn the copper quite black, and that the black color would not be on the surface, but penetrate the metals structure to an amazing distance (ie, it cant be polished out).
So... a little bromine might do the trick to turn regular fretwire into black beauties.