Well they do .. but I suspect that it was a phrase older than military aviation - which was barely twenty years old at the time of the Hobbit .... of course ships would have gone of into the blue as well...disappearing where the blue of the sky met the blue of the sea...
Edit: I looked this up in Brewers' last night and no joy
.. Maybe you are right adn I need the 20th Century edition... but it just "feels" that it has been around for ever ..further back than WW1... but that could just be me...