Argentina has a "black" or "blue" market for USD and Colombia does not
ATM is about 6.25 pesos for the usDolaar. The blue rate is today (thanks Jackson) about 9.65 pesos for the dollar. You might pay a $15 USD lunch by credit card and it is not worth the hassle for a a small amount of money. For a US$1,000 exchange it is worth the hassle.
There are a few member of this site who earn the living in pesos who likely would love to buy us Dollars. You mighr get a PM offering to buy your dollars at a rate about the blue rate in an easy and painless manner.
If you employ TL as a tour guide, he will walk you through a blue market exchange. If you use "Silver Star Taxi" for airport transfer, Fred will help you exchange money has part of the luxury service he provides. Both Fred and TL have been doing this for years and are both trustable to help you with an exchange. Or you may do it on your own.
Thanks for the "on the street" intel