Hearthstone purchases on iOS devices are tied to a single Apple ID, regardless of how many Blizzard accounts it's associated with.
If you attempt to purchase an item from a Blizzard Account, and you've already purchased that item from another Blizzard Account with the same Apple ID, you will see an iOS message that says, "You've already purchased this. Would you like to get it again for free?" This message is not accurate, and clicking OK will lead to a purchasing error.
To solve this problem, either buy the product through the Blizzard Shop on your PC, or use a different Apple ID.