Blue-Gray Macules After Melanoma

An 84-year-old man with a history of melanoma of the right forehead complicated by in-transit metastases to the scalp presented to the dermatology clinic. Seven months prior, treatment with two immunotherapy agents had been started, which had resulted in shrinking of the metastatic scalp nodules. On physical examination, there were multiple blue-gray macules on the scalp, as well as an unhealed ulcer from trauma. Dermoscopy-guided biopsy of four macules revealed scattered melanin-laden macrophages within the superficial and middle dermis without any evidence of residual melanoma. Immunohistochemical staining of a biopsy sample for markers of melanoma was also negative. What is the most likely diagnosis?