Any purchases made while playing Hearthstone on your tablet or phone will pull from the payment you have set up on your Apple ID or Google Play account or Amazon account, not from your Blizzard account. In order to use your Blizzard Balance to make purchases in Hearthstone, you will need to log into the game from your PC or Mac. Any purchases made on the PC/Mac version of the game will also be available to you on the tablet and phone versions of Hearthstone.