If you are really interested in keeping blue rings, do a search here for past threads...we have discussed the heck out of it !
There is ALWAYS a chance of being bitten. The threat of death also hangs above the situation. My advice? Get a 55 and a bimac...gain a bunch of years of experience with cephs, and with other venomous animals prior to trying to maintain a blue ring.
Yes. Just because an animal can live in an aquarium doesn't mean its less dangerous to keep. There are a lot of people who keep "hot" pets, and a lot of them are very experienced with their animal's non-poisonous relatives and know how they can keep their pet while minimizing the chance of getting injured. Keep in mind that you still have to open the lid to remove uneaten carcasses and do other regular maintanence.
Keep in mind that the blue-ring octopus is one of the most venomous animals in the world. People bitten are usually dead within a few minutes--long before the ambulance arrives. For contrast, I know ranchers who've been bit by brown recluse spiders and not even know it.
There are a few people that have kept blue-rings successfully, but the majority hasn't. Why take the risk? It isn't just your life, but chances are it won't live long and will be a waste of money. If you spend that money on a tank upgrade instead, a bimac would probably make you much happier.