How does that work? I take it you can't just buy dollars at the official exchange rate? If that's the case, than what's the point of having it?
From what I understand, No, regular individuals are not offered the official rate. It is also called the interbank or mid-market rate;
Interbank cause, it is mostly used when banks are exchanging money among themselves or similar high volume transactions.
Mid-market cause, it's the average between the buy and sell price.
If a regular individual wanted to buy USD, banks would charge a higher rate than what they sell at.
Banks also use markups, which are extra cost on the purchase of a currency. The percentage markup of a bank is usually not public.
The official rate is highly regulated to control the perceived state of the economy and avoid reflecting the rate of a crisis, that may one of the points of having it. Such regulation is not present in the black market.