Black lightsabers were lightsabers with black blades. These were designed by emo Jedi Padawans who thought that since black was cool, that it would make for a cool-looking blade. These Padawans had never seen a black light before, and failed basic elementary science classes, as everyone knows that black does not emit light. The Sith Underlord Darth Cold-Dead-Hands tinkered with the concept at one point in his career.