We just returned from BsAs (Dec. 1-9), and it was simplest to just walk down Florida Ave. to get pesos at very close to the blue rate, which was ~12.5-13 while we were there. There seemed to be a person every 10 feet or so saying "cambio cambio", and we just asked what their rate would be for the amount of money we wanted to change, they told us (or showed it on a calculator), then we kept walking if we thought we could do better. It seemed the rate increased ever so slightly the further down the street we got, from 12.6 to 12.8 the few times we went. Once we agreed on a rate, we followed the cambio guy into an "office" off of the street, gave them our $100 USD bills, they gave us the agreed-upon number of pesos and we left. Easy as pie.
Even easier, we found out, was that most shops and every restaurant where we ate readily accepted USD, and during our trip, they mostly offered a 12 exchange rate, a few offered 13 exchange rate. So we'd go to dinner, pay with a $100 bill, and get change in pesos to use the next day, and repeated. One shop would only offer 11, so we used pesos there.
If we ever go back, I'd head to Florida Ave, exchange $500 USD, then just spend USD at restaurants and shops and get pesos as change to spend (taxis, etc.)
The main point being is that you'll always do MUCH better than the official exchange rate with USD cash, it's accepted almost as widely as pesos, and you'll sometimes get a better rate in a shop or restaurant than you can do on Florida Ave.
Also, fyi, no one seemed overly concerned about the condition of the $100 bills we brought. They can be sticklers in Burma and sometimes in Thailand, so I brought new, sequentially numbered $100s, and the cambios were completely non-plussed!