Beware black at the border

Dermatoscopic black is produced by melanin at the surface of the stratum corneum. 

Melanin is a very efficient pigment and it absorbs all light reaching it. When there are no structures (such as epidermal, dermal cells or particles) between it and the observer to reflect or scatter incident light, it will appear black (Figure 2).

Malignant cells spread outwards early in the growth of melanomas (radial growth phase) and upwards towards the surface of the skin which is bounded by the stratum corneum. This is known as pagetoid spread. 

Peripheral black dots and clods are generally produced by melanoma cells or nests respectively, close to or at the level of the stratum corneum due to such pagetoid spread.