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.
what is the meaning of the idiom out of the blue
The idiom “out of the blue” means that something happens when you are not expecting it to or something happens suddenly.
anything else about the idiom out of the blue bestides the meaning and origin
There are several words and phrases that you can use that would be synonymous with the idiom “out of the blue.”

Question: what are the alternatives
Self-contained rewrite: what are the alternative words and phrases for the idiom out of the blue