A malignant blue nevus of the right upper arm with hematogeneous lung metastases is presented. Histological examination showed that the tumor was composed of spindle, dendritic, and globular cells with hyperchromatic and polymorphic nuclei, atypical mitoses, and tumor cell necrosis. There was no proliferation of atypical melanocytes at the dermoepidermal junction. Histologically, malignant blue nevus should be distinguished from benign cellular blue nevus, primary cutaneous malignant melanoma, cutaneous metastases of malignant melanoma, and clear cell sarcoma.