Just how rare are these creatures? According to this article on BBC it is about one in two-million.

"The probability of the creature having this colouring is widely touted as being one in two million. The University of Maine Lobster Institute is among those to have quoted these odds in the past, though its executive director, Rob Bayer, admits they are merely a "guess".

How do they work it out?

A rough analysis of overall catch tonnages suggests that about 200 million lobsters are caught in the North Atlantic every year, he says. That equates to 100 blue ones turning up in a year on average, if the one-in-two-million odds are correct.

Original BBC article link: http://www.bbc.com/news/magazine-36369687