Out of the spectrum reflected in a puddle of rainbow water by the two moons, the least commonly occurring color is that on that's not quite green, but verging on pruple, though darker but not black...
Shut up. It's blue, asshat.
When people on the Avantsphere speak of an anticipated, near expected "doomsday" they need a new term. Everything is Doomsday. At least three raider babies born a day get named Doomsday or some near variation. So many probable and possible doomsdays have come and left they can't be kept track of by even the most dedicated of scribes.
People talking about the end--the true end of life on the Sphere--they've taken to a common knowledge term calling it The Blue Age. It's also known Aoi-jidai in an uncommon though trendy tongue.
There are scattered origins claimed to be behind the now well-known phrase, but none of the stories lined up. For now, it's being called a phenomenon of language, generated through repetition and popularized due to creating an image most people can now associate with what the true end of the Avantsphere means.
The wastes rarely come in the color blue, to the point where some just consider it a myth of a trick of the eyes. So, as it goes, when the world turns blue that will be when it's all over.
The common image is one of a blue world wherein nothing lives, like a darkening or life and known existence. There is, however, an alternate interpretation some claim the true meaning of The Blue Age is trying to intone--that when The Blue Age comes to pass, it will signal the start of something new and never seen, beyond the comprehension of the Avantsphere's current inhabitants. Right now, all they can picture is death until it ends. Maybe The Blue Age will be a dawning of a new time they could never imagine, one wholly of peace.
Nah, it will probably be a diseases that turns everything blue and gives you the runs and shivers until you die.
But who knows. A number of dudes are convinced The Blue Age is coming, even if not in their lifetimes.