Well, it seems that the US application for "Hearthstone" claims priority to February 28, 2013 (thereby predating the PAX announcement) via a related trademark filing in Swaziland! See:
http://tsdr.uspto.gov/#caseNumber=86...e=statusSearch
Under 44(d), Blizzard has up to 6 months from the earlier foreign filing to file in the US and retain the earlier filing date as a US priority date. There does not seem to be any apparent reason to file in Swaziland first unless Blizzard intentionally wanted to delay filing in the US, thereby "hiding" the name from public release in the US Trademark database until it could make the press announcement at PAX.