I am going to be in BA in early November. I know there is a big difference between the official exchange rate and the dolar blue rate for US dollars.

Where do I go to exchange US dollars for pesos at the dolar blue rate? Is it a legal transaction? Or an illegal activity? Is it better to exchange 100 dollar bills or 20 dollar bills? What about the actually money? I know about no rips or writing on the bills but what about dates of issuance? Would it be beneficial to get the new 100 bill?

I will be in the micro center area of BA.

Thanks for any information.