What exactly is melanosis cutaneous fibromatosis?

Disease. Depends where the lesion is. Likely it is a benign tumor (fibroma) that has skin color changes (melanosis) that need to be followed and biopsied on a regular basis. Autoimmune disease might also be a contributing factor. Ur dermatologist could best answer this.
Darkened skin tags. Discoloration can result from trauma or just seen in more darkly pigmented individuals. Although skin tags are usually flesh colored they can be brown or black in appearance.