That is using the 'official' rate, which the rate the central bank forces(if you transfer money to argentina, the central bank will keep the dollars and give you pesos) you to sell your dollars to(about 8.05). But you cant buy dollars from the central bank, so you have to get them 'illegaly', which is between 11 and 12 pesos per dollar today.

Thats what the OP refers to with black market exchange.

Disclosure: I am also Argentinian