No, not black light. BACK. As in, if you put a work light behind the surface you're applying finish to, at roughly the same height as your eyes or perhaps a bit lower, that light will reflect off the wet finish into your eyes, so any spot that you've missed will show up darker, and any little piece of sawdust or hair will distort the liquid surface of the finish and distort the reflection.
Try it! you'll see a difference right away.