expression, noun
out of the blue; bolt from the blue; complete surprise
Idiomatic expression for something sudden and unexpected, often used when someone says or does something abruptly without context.
彼は藪から棒に「結婚しよう」と言った。
Out of the blue, he said, "Let's get married."
藪から棒な質問で驚いた。
I was surprised by the sudden, unexpected question.
突然 is a general adverb/adjective for 'sudden'; 藪から棒 is a more colorful, idiomatic expression emphasizing the lack of warning or context.
青天の霹靂 is a more formal, literary idiom for a bolt from the blue; 藪から棒 is colloquial and often used in everyday speech.
Literally 'a stick from a thicket', evoking the image of a stick suddenly poking out of a bush. The exact origin is uncertain, but it vividly conveys something abrupt and unexpected.