Diagnostic black dots on the surface of the lesions of chromoblastomycosis are the preferred areas from which one may visualize, by direct light microscopy, the classic, spherical, fungal elements of the causal organism. These elements often are called "Medlar bodies" or "copper pennies."
The black dots represent upward epidermal migration of products of the inflammatory response plus hemorrhage as a result of host-parasite response in the dermis. They are formed from microabscesses in the dermis, which lead to the surface of the skin. Rounded fungal forms are seen in or near the "black dots," while hyphal fungal forms are seen in the adjacent, chronologically older horny layer.
Zaias N, Rebell G. A Simple and Accurate Diagnostic Method in Chromoblastomycosis. Arch Dermatol. 1973;108(4):545-546. doi:10.1001/archderm.1973.01620250033009