"Out of clear blue sky" is an idiomatic expression that is often used to describe a sudden event or occurrence that happens unexpectedly and without warning. Some of the synonyms for this phrase include "out of nowhere," "out of the blue," "out of thin air," "unexpectedly," "out of left field," and "suddenly." All of these phrases carry a similar meaning and can be used interchangeably depending on the context of the situation. Whether it is a surprise promotion at work or an unexpected breakup, these synonyms provide effective ways to convey the suddenness and unpredictability of such events.