I'm on the fence with this one... I'm leaning towards Scutus but Bongo's confidence in his Stomatella ID has me wavering. I should also say that my opinion is based on books (photos and descriptions) and Google images solely, as I've never seen a live Scutus or "black" Stomatella (plenty of gray S. varia though).

You may want to get a definite ID on this one as the description in "Invertebrates: A quick reference guide" by Sprung states the Scutus eat coral tissue and are not reef safe. I'd post this photo (and/or clearer ones) in Marine Depot's Invertebrate forum where Dr. Shimek could throw in his 0.03 cents. And please let us know what he says!

Tom