Congenital Melanocytic Nevus: This is a common benign skin lesion that can present as a pigmented blister or nodule at birth. The presence of melanin pigment within the blister supports this diagnosis, as congenital melanocytic nevi are known for their melanocytic origin.

Mongolian Spot: Although typically found on the lower back, a Mongolian spot could potentially appear as a pigmented lesion elsewhere, including the arm. However, it's less likely to present as a blister.
Café-au-Lait Macule: While not typically blister-like, a café-au-lait macule could be considered if the "blister" description is interpreted loosely as a pigmented lesion. These are benign and common in newborns.
Dermal Melanocytosis: This condition involves the presence of melanocytes in the dermis and can present with bluish or brownish pigmentation. It's less common but could be considered in the differential for a pigmented lesion.

Malignant Melanoma: Although extremely rare in newborns, it's crucial not to miss this diagnosis due to its severe implications. A biopsy would be necessary to differentiate it from benign lesions.
Infectious Lesions (e.g., Herpes Simplex Virus): While less likely given the description of melanin pigment, certain infections can cause vesicular lesions. The presence of systemic symptoms or a cluster of lesions would raise suspicion for an infectious cause.

Epidermal Nevus Syndrome: This is a rare condition characterized by epidermal nevi that can be associated with various systemic abnormalities. The presence of a single blister with melanin pigment would be an unusual presentation but could be considered in a broader differential.
Phakomatosis Pigmentovascularis: A rare condition characterized by the combination of vascular and melanocytic lesions. It's an unlikely cause for a single blister but is included for completeness in a broad differential diagnosis.