The blue black rule
The blue –black rule was proposed to identify pigmented nodular melanomas, which often appear as symmetric papules or nodules.5 This rule is based on the presence of blue-back color, which is defined as the presence of these two colors in at least 10% of the lesion surface area.
The presence of the blue-black color in a papule or nodule demonstrated 78.2% sensitivity for melanoma, which improved up to 84.6% when the observer also sought to look for other melanoma-specific structures. Melanoma specific-structures are listed in Table 3. In the study Argenziano and colleagues evaluated the lesions for the presence of specific melanoma structures, including atypical network, negative network, irregular streaks, regression structures, and irregular brown structureless areas. Specificity for melanoma was 80.5% when using the blue-black rule only, 97.6% when using the classic melanoma-specific structures only, and 80.5% when using both criteria (i.e. blue-black rule and classic melanoma structures). In addition to detecting nodular melanoma, the presence of blue-black color can also aid in the detection of other heavily pigmented skin malignancies, including pigmented nodular BCCs.