This sample of chalcostibite is displayed in the Smithsonian Museum of Natural History. Chalcostibite is a sulfide mineral of copper and antimony with the composition CuSbS2. This sample is about 7x7 cm and is from Rar El Ans, Cherrat, Morocco. The sample is described as chalcostibite altering to azurite with calcite.
The normal color is stated as iron gray. Presumably the blue color here is from the forming azurite.