Conversation so far:
where did the phrase out of the blue come from
The idiom “out of the blue” evolved from an older idiom. The older idiom used was “a bolt from the blue” or “a bolt out of the blue.”
what was the cause of the idiom out of the blue
This phrase related a situation to a lightning bolt appearing suddenly from a clear blue sunny sky when it was not expected.

Question: what is the meaning of the idiom
Self-contained rewrite: what is the meaning of the idiom out of the blue