out of the blue／a clear sky
without any warning；unexpectedly出其不意地；忽然
You may want to spend time with a longlost friend who drops in out of the blue．如果有一位久已失去联系的朋友突然来访，你会很想同他多呆一会儿。
The cowboy thought he was alone but suddenly out of a clear sky there were Indians all around him．牛仔以为只有自己一人，谁知忽然间身旁出现了许多印第安人。