Anorectal malignant melanoma is a rare aggressive neoplasm, and its amelanotic subtype lacks visible pigmentation. Colonoscopy identified a reddish, slightly blackish, elevated 20-mm lesion in the lower rectum. Narrow-band imaging showed an avascular area with dilated vessels. Magnifying endoscopy with crystal violet staining demonstrated a non-structural pit pattern with residual type I pits. Histopathology revealed a submucosal tumor composed of sheets of atypical round to short spindle-shaped cells. Residual crypts corresponded to the endoscopic findings. Immunohistochemistry positive for HMB-45 and S100, with negative Fontana-Masson staining, confirmed amelanotic melanoma. The faint blackish discoloration was attributed to iron-positive pigment associated with intratumoral hemorrhage rather than melanin.
