Hotels are aware of the blue vs. official USD spread, so the better hotels which have good occupancy have adjusted their USD rates to take this into account. In any event, its probably a good bet to still book a post paid rate because given the recent volatility of the Blue rate there is a decent chance that you will be able to trade your Dollars for a nice rate and save some money... the only thing that is for sure is that the 21% VAT you save by paying with a foreign credit card does NOT offset the rate difference, so paying your hotel with a ccard is something to try to avoid at pretty much all costs.