Not sure about the black prupleheart, KP, but the cocobolo I referred to just lost its vibrant color when I cut it, planed it or sanded it. But in a few days, the color was as vibrant as ever. Don't know if you would call that lightening or darkening. Sorry for the confusion.
What gives purpleheart its color, anyway? and what makes it change to purple? Does more of this pigment make it darker? Is your piece just extra rich in pigment?