"Out the clear blue sky" is an idiom often used to describe something happening unexpectedly or suddenly. There are many synonyms that can be used in place of this phrase, some of which include "out of nowhere," "out of the blue," "suddenly," "unexpectedly," "abruptly," "without warning," and "out of thin air." Each of these phrases can be used to convey the same sense of surprise and unexpectedness that "out the clear blue sky" does. When writing or speaking, it is always important to choose the right synonym to fit the context and the tone of the conversation.