Yeah, I’m thinking a black sponge of some kind, though tunicate is a good guess (particularly with what looks like a huge siphon hole in the center of the black mass). If you look at the left side of the mass just above the siphon hole, however, you can see a tansparent, black-tinted tube that would seem to be a strong indicator that it’s a sponge. Either way, it’s a harmless filter feeder, and it’s got some nice coloration.