Easy enough to avoid using the black market if that's your goal: just change money legally, at the legal rate. Only drawback is that everything becomes 50% more expensive. If you don't mind that, you're in fine shape. Note that "legally" includes all debit and credit transactions.
I'm having difficulty seeing the advantage of POS transactions using a bank card. You'll still need to carry cash, since as you've noticed you don't really have the option of paying by debit card in a lot of the places you'll end up. Any reason you're so determined to do things the most expensive, least convenient way?