We report that a self-trapped black optical beam that is spatially and temporally incoherent forms spontaneously in a nascent photopolymerization system. The black beam inscribes a permanent cylindrical channel, which prevents the propagation of visible light even under passive conditions (in the absence of polymerization). The finding opens a powerful new mechanism to manipulate light signals from incoherent sources such as LEDs through selective suppression of light propagation. This contrasts with approaches employed by photonic crystals and optical waveguides, which concentrate and guide light intensity within spatially localized regions. The self-trapped black beam forms when a broad incandescent beam bearing a negligible depression was launched into a photopolymerizable medium. Because of refractive index changes caused by polymerization, the depression narrows, deepens, and continually rejects the visible spectrum of light until it stabilizes as a black beam that propagates over long distances (≫ effective Rayleigh range) without significant divergence. As refractive index changes due to polymerization are irreversible, the cylindrical region occupied by the self-trapped black beam is inscribed as a black channel waveguide in the medium.