The current (official) exchange rate for USD to ARS is around 1:5, but I've heard that due to the fluctuation recently there may be a way to get a better value for USD in Argentina.
Fist of all a short disclaimer: The blue market is an unoficial market not recognized by federal authorities and changing money there is illegal. Thousands of argentineans use this market everyday as buying dolars in the official market is almost prohibited by the government .
I will try to answer your question without entering into the details of the current political and economical situation which is currently driving Argentina.
As you pointed out, there are currently 2 very different rates for the Argentinean Peso (ARS)
According to Yahoo Finance the official exchange between $1 Dolar and the Peso is $4.96
What you mention in your question is what is known as the Blue Market
As stated in the disclaimer, currently it is very difficult for argentineans to buy foreign currency which has made the price go up in the secondary market.
You can find the current value per $1 USD on many unoficial websites like PrecioDolarBlue. As for today $1 USD is about $7.50 pesos
Where can you trade the Blue Market?
People selling the blue dolar are called "arbolitos" (little trees) and places were you can buy/sell the blue are called "cuevas" (caves)
While you are in Buenos Aires you can ask any cab driver, shop owner, hotel personal, etc, for the closest arbolito or where to trade the blue market and they will be able to point you in the right direction.
Disclaimer again: Remember it is illegal (although AFIP (Argentinean IRS) does not seem to be chasing buyers)