Blizzard has registered a domain for something called Project Blackstone, a domain which was once owned by a Texas resident who let the registration expire last year.
According to WHOIS records dug up by Fusible, Blizzard nabbed projectblackstone.com on November 26.
At this time, the domain does not resolve to a web page, so it's unclear what Blizzard intends to do with the domain name.
The only Blackstone we know of in relation to Blizzard is Blackstone Span, a bridge in WoW which connects Tol Barad and the Tol Barad Peninsula.
So while the domain could eventually become something World of Warcraft related, such an expansion, it could also just be a a domain grab.