Meaning / Explanation:
According to our knowledge it used to define a thing that happen suddenly or on a unexpecting time. It can anything like the rain started at a unexpecting day or time when the sky is clear and blue. Out of the blue is also used when you are not ready for any situation or not prepared properly.
Usage / Example:
Did you know that our boss send you at a holiday?
Reply: I don’t know It was completely out of the blue.
In simple definition:
1. Unexpected or unforeseen source: criticism that came out of the blue.
2. At a completely unexpected time: a long-unseen friend who appeared out of the blue.
Out of the blue I feel that she is avoiding me, ignoring me and playing hide and see with me. I'am totally clueless.