From everything I've heard — and from leaks we've seen so far — Windows Blue will include both new features and fixes, technically putting it outside the strict "service pack" category.
In the not-so-distant past, Microsoft's Windows team did allow for new features to be included in a service pack. Remember Windows XP SP2? That version of XP really should have been called something other than "SP2." But the chief of Windows at that time, Jim Allchin, made a conscious decision to use the SP2 nomenclature to prevent any consumer or business customers from holding off on deploying this key, security-focused version of Windows.
After XP SP2, Microsoft's Windows team moved to a model via which service packs only included fixes, not features. I believe this has been the team's policy ever since. Any Microsoft historians: please correct me if I am wrong.
It will be interesting to see if Microsoft ends up calling Blue's successor "Windows 8.X" (with x being some number greater than 1) or "Windows 9." In most ways, this is an arbitrary, and marketing-driven decision. If Windows 8 ends up perceived by the general public more positively than it is currently, Blue's successor may end up as an 8.X release; if it doesn't, Microsoft could end up going with Windows 9 just to distance itself from Windows 8. (This would be very similar to Microsoft's decision to move away from "aspirational" names like "Windows Vista" to "Windows 7," once Vista was poorly received in the market.)
The Windows client and Windows Phone teams are moving toward bringing their programming models and developer tools more into alignment. The overarching goal is to enable developers to write once and run on any Windows variant with less code-tweaking required. But the phone and PC/tablet variants of Blue are two different things, regardless of how much Microsoft marketers may attempt to blur the lines.
As to reports that the Windows team is working on another, non-Blue version of the operating system that could be released in October of this year, I'm wondering whether this might be a reference to the operating system at the heart of Xbox Durango. After all, that OS is supposedly based on the Windows 8 core, as was/is Windows Phone 8. (While the core is the same, the rest of the OSes, in both the phone and console cases, is different and built by the Windows Phone and the Xbox teams, respectively.)
But the most likely explanation is these reports are misguided or confused, based on everything I've heard.