Well, Windows Blue certainly looks like it's a thing, as today a new job opening appeared on the Microsoft Careers website, which talked lots about Windows Blue and Windows Phone Blue.
"... The Core Experience features are the centerpiece of the new Windows UI, representing most of what customers touch and see in the OS, including: the start screen; application lifecycle; windowing; and personalization. Windows Blue promises to build and improve upon these aspects of the OS, enhancing ease of use and the overall user experience on devices and PCs worldwide."
From the job opening, it appears Windows Blue is going to focus on the Windows UI (Metro) and will build and improve upon it. Hopefully this means more customization on the Start Screen, I want folders and smaller icons!
Windows Blue appears to be a wide-spread update that Windows 8 and Windows Phone will receive, there is no exact date for the upgrade as of yet, but many are speculating something to be released in mid-2013.