Dwest,
The pesetas, and other currencies that you refer to, are not really black market ones. They are called dinero negro(black money), because it is money coming fom incomes for which people don´t pay the due taxes. Many people have this money in hidden accounts, in cash, ..., and are hurrying to use it to buy flats or whatever, because soon their notes will be worth nothing, thus inflating prices.
As you see, this has nothing to do with tourists or their money.
Ignacio