Evicting tenants is not the most straightforward process in Argentina, so if a landlord lets a flat "informally" (as is the case in most touristic, AirBnB-like rentals) he/she is expecting a much bigger profit (USD or pesos at the blue rate), otherwise it doesn't make real sense.

On the other hand, I know of many people who get their pesos at a cueva (blue rate) and pay for their hotels in cash. That's OK and it's pretty easy to do.