Samples of peeling black crusts from modern and historic buildings in Campeche, Mexico, from a gravestone on the island of Dom Khon, Lao, and from the Anglican cathedral in Belize City were analyzed microbiologically, by scanning electron microscopy plus electron dispersive spectroscopy (EDS) and for pigment composition. In all cases, the surface was covered by a thick mat of cyanobacteria with dark brown sheaths. These were filamentous organisms of the genera Scytonema or Fischerella/Mastigocladus, except for one sample, where coccoid cyanobacteria of Subsection II were predominant. Fungi were not present at all sites and, where seen, were not the major biomass. High scytonemin:chlorophyll a ratios correlated with the dark pigmentation of the cyanobacterial cells and indicated the stressful conditions under which these organisms were living (high temperatures and ultraviolet levels, frequent desiccation). The absence, or low levels, of sulfur in the biofilms confirmed that there was little urban pollution at the sites and the EDS analysis showed that the black coloration was caused solely by cell pigmentation; no dark-colored elements were present at high concentrations. These results demonstrate that, unlike chemically formed thick black crusts found in polluted atmospheres, thin black crusts (which could be called patinas) in clean environments may be predominantly composed of filamentous cyanobacteria.