While the us$ is preferred over many other countries currency since the us$ is more readily convertible into any currency someone may need, there is no real special "black market" value for the us$ exchange as there was years ago. You will receive nearly the exact same rate to exchange money from the cambio or bank as you would from some person asking you to exchange money on the street.
And please remember that there are many reasons not to ever exchange money on the street... from counterfeiters to scammers to con artists...
For any potential "benefit" you may think there might be for a better exchange rate, the axiom "if something sounds too good to be true, it probably is" should be foremost in your thoughts...
