A full moon rises over the Reflecting Pool on the National Mall in Washington, D.C. on September 30, 1993 in this background photograph. It was the second full moon in that same calendar month referred to as a blue moon. Since this happens infrequently, the term has become an idiom for other events that seldom happen. This idiom and other examples are listed below.
Once in a blue moon: very seldom
The wife complained that her husband brought her flowers only once in a blue
moon.
The parents allowed their children to go to the movies once in a blue moon.
Out of the blue: unexpectedly, unplanned
I hadn't seen my cousin in ten years, so his phone call yesterday really came
out of the blue.
While brainstorming for possible solutions to the problem, out of the blue
Rick came up with a great idea.
Blue collar workers: laborers and factory employees,
sometimes union workers (in contrast to white collar workers, who are
professionals and office workers)
The blue collar workers planned a demonstration to protest the
exaggerated high salaries of the company president and vice-president.
The government reported that blue collar employment rates went up last
month.
Blue chip stock: a share of a large company that has high
value
For the third day in a row, the blue chips were lower when trading ended
at the stock exchange.
Her financial advisor recommended that she buy blue chip stock as an
investment.
Blue in the face: pale from exhaustion or frustration
Her jokes made me laugh so hard I turned blue in the face.
The students argued until they were blue in the face, but the teachers
still make them complete their homework.