Blizzard picked up a new domain last week and it's causing fans across the internet to ponder just what it may be in reference to. "ProjectBlackstone.com" is the newest Blizzard URL, though it's currently unassociated with any content. Whether Project Blackstone is in reference to an upcoming Blizzard game property, perhaps a book or other side-project, or just something Blizzard wanted to sit on because it sounds cool, time will tell.
Current fansite speculation is that Project Blackstone is a reference to the black soulstone of Diabo III, currently MIA as of the end of the ARPG's storyline. I'd consider this the most realistic guess, considering we know Blizzard is working on expansion content for the game. It does seem a little out of place compared to previous Diablo expansion titles like Hellfire and Lord of Destruction, however.
Other potential references are all over the place, what with Blackstone popping up in World of Warcraft in various places, or a potentially rework of one of Blizzard's classic titles BlackThorne. Many gamers are even jumping to the conclusion that Project Blackstone may be the final name for Blizzard's secret MMO title Titan. It all seems rather circumspect, to me, and I wouldn't be surprised if Project Blackstone is something completely unrelated, or perhaps a media campaign of some sort.
Hopefully we'll here more on the subject from Blizzard in the near future, but until then -- speculate away.