Black light (or ultra violet light) has many uses, the relevant one here being to detect certain substances that may otherwise be invisible to the naked eye. The implication of the name and image here are that a black light was used to see footprints (and something else?) that were not visible to the naked eye (but left traces of a uv detectable substance) and to be use as blackmail.

May 7, 2011 by Karmakula

i'm impressed, thanks a lot :)

May 7, 2011 by A Wikia contributor