A rapid change of the sort you describe nearly always reflects minor trauma, usually too minor to have been noticed at the time. The black part you're suddenly seeing is almost certainly a scab from this trauma. If you leave the mole alone for one month, or perhaps even less, the scab should fall off and leave the mole looking just as it always did. Unless this does not happen, in which case a visit to a physician is recommended, you should ignore it as you have done, quite properly, all this time.
Best.
Dr. Rockoff
RobertvV