Blue nevi are considered to be congenital lesions, albeit most of them are acquired in the sense that they are not apparent clinically at birth. Blue nevi commonly reveal clinical and especially dermoscopic features that are morphologically distinctive and allow a clinical diagnosis to be made with a high degree of certainty. In rare instances, however, blue nevi, especially when nodular, may be simulators of cutaneous melanoma from a clinical as well as a dermoscopic point of view [Ackerman 1994]. More important than this set of false-positive cases (clinically overdiagnosed melanomas) is the group of melanomas that are not excised because of the clinical and/or dermoscopic diagnosis of blue nevus, thus representing false-negative cases (underdiagnosed melanomas)..
Clinically, blue nevi appear as relatively regular, sharply circumscribed, monomorphous macules, papules, plaques or nodules with a uniform brownish-blue, blue to gray-blue or sometimes even gray-black pigmentation. Clinical variants of blue nevus, also referred to as diffuse melanocytoses, are the nevi of Ota and Ito that appear many years after birth on the face and on the trunk, respectively, and the so-called Mongolian spot over the sacrum that is present at or near birth. A less common type of blue nevus is the neuronevus blue Masson, clinically characterized as a gray to blue deep-seated nodule, usually situated on the buttocks, and involving the entire reticular dermis with extension to the subcutaneous fat.
Dermoscopically, blue nevi exhibit a homogeneous pattern with complete absence of local features, such as pigment network structures, brown globules or black dots within the diffuse homogenous pigmentation. This absence of local features and the presence of a well-defined border, usually without streaks, are criteria to differentiate blue nevus from melanoma, in many cases with a high degree of certainty. In some instances, however, the differential diagnosis between blue nevus and nodular melanoma is also dermoscopically difficult as identical dermoscopic findings may be present in both neoplasms [Pehamberger 1987, Kenet 1993]. A rather uncommon dermoscopic finding in blue nevi is the presence of diffuse hypopigmentations corresponding to more or less pronounced areas of collagenization in the reticular dermis of a fibrosing type of blue nevus. The nearly complete absence of local features within the homogeneous pigmentation of a blue nevus can be easily explained from a histopathological standpoint by the fact that virtually all blue nevi are situated mostly within the dermis with a small ‘grenz zone’ immediately beneath the epidermis.