Blue Nevus Associated With Acquired Dermal Melanocytosis on the Back.

Dermal melanocytosis includes various congenital and acquired pigmentary disorders characterized by dermal dendritic melanocytes. Blue nevi typically present as papulonodular lesions, whereas other dermal melanocytoses manifest as patches. This report describes a case of a blue nevus associated with acquired dermal melanocytosis on the back of a 46-year-old Japanese woman. The patient presented with a black nodule on a blue-greyish hyperpigmented area on the upper back. Histopathological analysis of the nodule confirmed a common blue nevus, whereas the adjacent hyperpigmented area showed features consistent with acquired dermal melanocytosis. Blue nevi and acquired dermal melanocytoses share a common pathophysiology involving ectopic melanocyte accumulation during embryogenesis. The coexistence of blue nevus and acquired dermal melanocytosis on the back is rare, highlighting the broad spectrum of dermal melanocytosis and the variability of its clinical manifestations. Recognition of such unusual presentations is critical for appropriate diagnosis and management.