I can think of only one reason to do this. You need to add just a little bit of light in a dark situation, and your flash unit at its lowest power setting gives too much light. Bouncing off a black surface would dramatically lower the level, so you could get fill in a situation where the ambient exposure is, say, 1/15th at f/1.4--in color, there's a good chance you'd get a nasty color shift because most "black" material is not really neutral in chroma.