Dematiaceous is a term to denote dark-colored. It is generally used to refer to a group of fungi, the Dematiaceous molds, that produce melanin

Melanin

Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

 in their cell walls. The melanin is responsible for the characteristic brown color of the fungal colonies when grown on agar.

Dematiaceous fungi show septated hyphae with irregular branching. They are responsible for a number of human infections, including Chromoblastomycosis

Chromoblastomycosis

Chromoblastomycosis is a long-term fungal infection of the skin and subcutaneous tissue . The infection occurs most commonly in tropical or subtropical climates, often in rural areas...

 and Phaeohyphomycosis

Phaeohyphomycosis

Phaeohyphomycosis is a heterogeneous group of mycotic infections caused by dematiaceous fungi whose morphologic characteristics in tissue include hyphae, yeast-like cells, or a combination of these.It can be associated with Alternaria....

.