Blanket octopuses are so called because of the transparent webbing connecting their dorsal and dorsolateral arms, which resembles a large a large flowing blanket.
The “blanket” is a defense mechanism, and a handy one too, as Blanket Octopuses do not have ink to ward off predators. Instead, the Blanket Octopus will unfurl its blanket, making it look significantly larger and intimidating, with the intention of scaring off whatever threatens it.
Wikipedia: Tremoctopus is a genus of pelagic cephalopods, containing four species that occupy surface to mid-waters in subtropical and tropical oceans. They are commonly known as blanket octopuses, in reference to the long transparent webs that connect the dorsal and dorsolateral arms of the adult females. The other arms are much shorter and lack webbing.