I also posted about this on a classical guitar forum but have you guys ever gotten a very dark purple sort of stain on the cloth after cleaning an ebony fretboard? The cleaning was just with warm water on a coarse microfiber cloth and lots of elbow grease, especially along each fret. It's my everyday player (classical) guitar and it's been about a year since its last through cleaning, although I wipe everything down each day after playing.
Usually the schmutz that comes off a fretboard to dark gray to black but this was a definite purple hue, about like the stain a real inky Malbec wine would leave on a wipe tablecloth. I just can't imagine where that coloration would originate from wood, string and fingers.
I'm sure the fretboard is ebony, it's slightly stripy with brownish mineral streaks here and there but overall fairly black. I can't imagine it was dyed, partly because it's not solid black and partly because it was (back in the day) a fairly pricey guitar from the Kohno shop. Otherwise I'd figure I had just taken some dye of an "ebonized" rosewood 'board or something like that.