If you say that something happens once in a blue moon, you are emphasizing that it does not happen very often at all.
[emphasis]
Once in a blue moon you get some problems.
See full dictionary entry for moon
blue moon in British English
noun
1.
the second full moon occurring within a calendar month
2. once in a blue moon
blue moon in American English
a very long time
Idioms:
once in a blue moon
Word origin
< a rare atmospheric condition in which layers of smoke, dust, etc., as from forestfires, volcanoes, etc., cause the surface of the moon to appear to be blue
Examples of 'blue moon' in a sentence
blue moon
ONCE in a blue moon you see a goal like that.
The Sun (2011)
A rare blue moon sees a leading role for you within a gifted team of friends.
The Sun (2012)
The origin of the term is actual blue moons, seen under certain meteorological conditions that can also turn the Sun blue.