Question about “Inconstant Moon” by Larry Niven

The characters realized there had been a massive solar flare when the moon became spectacularly brighter. If the sun became hotter in “Inconstant Moon,” its light (and thus moonlight) would be bluer.

Why didn't anyone in the story recall the phrase “once in a blue moon”? They might have realized this had happened before.

On the other hand, there are other reasons for a blue moon.