The first time I encountered these nudibranchs I confused them with some very dark Common Sea Hares. Turns out it is not uncommon to find them hiding in similar crevices in rocky, protected areas. The white tips on their rhinophores are a bit of a giveaway — though apparently, the darker version of the  Red Dendrodoris or Dendrodoris Sp.#1 is sometimes mistaken for the Nigra.  To about 3″ or 75 mm. But many of the ones I have spotted were much smaller. Some of the younger ones can be unexpected colours (red or yellow), and almost translucent. For more info, please refer to Cory Pittman and Pauline Fiene’s Sea Slugs of Hawaii.