Boaz Elkayam made at least one Link Removed, and Lane Poor's headless basses had black epoxy fingerboards with "steps" cast into them for frets. Both looked completely black, neither sounded like a traditional fret.
I seem to recall that Danny Ferrington made a bass for Don Was that had ebony frets, though this was done for sound, not for looks.
Pics of the last two from the web:
I seem to recall that Danny Ferrington made a bass for Don Was that had ebony frets, though this was done for sound, not for looks.
Pics of the last two from the web: