Jet black frogfish are surprisingly common considering they don't really look like anything specific. Obviously its a fairly effective disguise generally speaking as black froggies are usually very large. You can usually see some starry patterning on their skin too.
This chap was huge and if you see look above her sinatra like blue eye you can see a white V which is the lure she'll use to attract food. These guys are also called anglerfish and use a lure (properly called an Esca) on a fine filament to lure in fish to eat. I'm hoping to see a few long lure frogfish soon so one of my goals is to get a better picture of the lure in action.