We first heard about Windows Blue a few months back, where it was revealed via unnamed sources that Windows Blue would be an update-like service pack thing for Windows 8. Today, more information has been released, revealing that the update will include UI changes to Windows 8, and even more tile sizes to further customize your setup.
Also, Windows Blue will be the first of many yearly upgrades, as Microsoft want to start issuing updates more frequently. It appears Microsoft will start the frequent updates with Windows Blue. It is also said that Windows Blue will include a new SDK for developers to update their apps, as apps developed for Windows 8 will no longer be accepted in the Windows Store.
We expect Windows Blue to include more things, but since Microsoft are staying tight-lipped on Blue, we can only speculate until official information is released.
Windows Blue is expected to launch in mid-2013.