Microsoft referred to its next PC operating system, Windows 8.1, as "Windows Blue," in some of its earliest stages of development. At the time, there was talk of a "Windows Phone Blue," the next major version of Windows Phone.
The name made its appearance on the web again, with talk of a 2014 launch. "Windows Phone Blue" succeeds not just the Windows Phone 8 GDR2 update that's around the corner; but also the GDR3 update due for late-2013, that introduces support for higher resolution screens, making devices such as Nokia's fabled 6-inch phablet a reality.
"Windows Phone Blue," going by the nomenclature precedent set by Windows 8.1, could be named "Windows Phone 8.1," but given that it will launch next year, and introduce a few pretty significant UI changes, it wouldn't surprise us if Microsoft christens it something less subtle, like Windows Phone 9.
"Windows Phone Blue" could introduce better multi-tasking (probably to combat iOS 7 and its focus on that), a proper notification center (don't be surprised if Microsoft calls it a system tray), which is yet another design idea drawn from Android via iOS 7, and a handful of new stock apps.