This species is probably Hapalochlaena lunulata, although there are several species known as "blue ringed". According to the California Academy of Sciences:
But don't worry too much. I found this website offers some helpful advice for divers bitten by the Blue Ringed:
When the tiny Blue-ringed Octopus is threatened, its faint blue rings become bright and vivid. The color warns predators to back off, and with good reason — it’s one of the most deadly venomous animals on Earth.
The octopus bites prey with its jaws, then injects a potent venomous saliva into the bite wound. This venom contains tetrodotoxin, one of the most potent neurotoxins known. Human death from heart and respiratory failure can occur within minutes.
Mouth to mouth resuscitation can keep the victim alive and the poison gradually wears off after 24 hrs, apparently leaving no side effects. Such patients that have survived such an experience talk of the terror of lying immobile and conscious while people around them are convinced of their demise.
Feel better? Enjoy.