It strikes me as odd that they didn't just send you a DMCA notice. That's how I believe Blizzard has worked in the past. How did you receive this notice? E-mail would surprise me as well. I believe legal notices (I realize this isn't one, but meh) would only be sent by certified mail. Did you examine the e-mail headers? Further, when Blizzard contacts you, they generally include the name of the representative that sent you the email. That's been my experience at least.
--MyndFyre