They would only block your CC if they suspect it was used for fraud in your system. This usually presents in chargebacks from the card company on their end, and blocking the card is to prevent more chargebacks in the future.
Blizzard directly do not deal with CCs, instead they use dedicated company/solution, which is specialized into operating with payments. And they have several automatic methods for dealing with fraud or cheat technics. So your behavior have triggered some of these anti-cheat mechanics, so they have been applied for either your CC, your IP address or similar, this is usually temporary for some hours or so, but you will never be sure, if not test it back.
