This expression means ‘very rarely’, or ‘almost never’. A blue moon actually occurs more frequently than you might think, about every three years, but this is a relatively recent discovery. A blue moon was thought to be extremely rare and magical, and treated with appropriate reverence. Now we know that the appearance of a ‘blue moon’, an additional full moon that appears in the cycle (when you have two in one calendar month, for example), is relatively common, but the expression is still used to describe something that happens very rarely; and when it does it’s usually on a special occasion.
Example: I don’t drink, apart from the odd glass of champagne once in a blue moon.