Dark black discharges from the vagina that happen regularly, once a month, and last 3-5 days could be menstruation. We can’t be sure if those black discharges are real menstruations until we prove that you have ovulations. Blood-laboratory detection of progesterone and ultrasound detection of yellow body (corpus luteum) proves the ovulation. Lack of menstruation for 4 years suggests that you had decreased function of the axis hypothalamus-pituitary-ovaries (hypogonadism). Hormonal analyses (FSH, LH, estrogen, progesterone, prolactin…) and ultrasound examination of the genitals may supply an answer.
You may want to consult a gynecologist about this.