This is another question that really depends on how you look at it. Commonly found blacklights are just regular lights with filters or coats that block visible light. As a result, their efficiency is the same as regular light.  However, if you want your black light to glow brighter, you’ll need a light that has a high UV output, which would obviously increase energy consumption.  Moreover, as visible light is not able to pass, this energy is often converted to heat energy, causing the bulbs to get very hot.