Black light was an unstable form of light. It was presumably dark in nature, hence its name, very unstable, and difficult to contain. It might be found naturally in the universe but ultraviolet radiation could be converted into black light. The Sixth Doctor, who said it was not his area of expertise and called it "tricky stuff," said that some believed that if a black light explosion occurred, the effects could cause a chain reaction that would roll on until all matter in the universe was exhausted, and dimensional transference threatened the stability of the universe.
Black light's only known use was as a power source. It required an extensive and extremely sophisticated system connected to an aerial for the conversion of ultraviolet radiation to black light.
An unnamed race from the Andromeda Galaxy used it to power their technology. After the three sleepers retreated to Earth, they established an underground hiding place maintained by the L3 robot Drathro and put themselves into suspended animation, both of which were powered by a black light power system. A few years before the Doctor arrived, the system had begun to breakdown and Drathro, trained only in installation and maintenance, was unable to correct the problem, nor were his two assistants able to. Despite Drathro’s stubbornness, the Doctor was able to prevent the first black light explosion from occurring. (TV: The Mysterious Planet)