I did it and it was by far one the dumbest things I have ever done in my life. Black henna does not fade so the only way of getting rid is either shaving your head, wear a wig until the black grows out and you have chopped it off, or bleaching your hair which at that point, you might as well shave your head because you will kill your hair.
If you must have black hair, go to the salon and have the professional do it.