The black substance created by a yellow Bunsen burner flame is soot. Soot is a carbonaceous material produced by the incomplete combustion of organic materials. It is composed of fine particles of carbon, ash, and other compounds. When a Bunsen burner flame is not adjusted properly, it produces a yellow flame that is indicative of incomplete combustion. This incomplete combustion results in the formation of soot, which can be seen as a black substance on the burner nozzle or on the surface of the object being heated.