Pretty much anywhere that isn't a bank branch will exchange at blue rate. Of course under the table, outside they'll advertise the official rates but nobody except maybe the most naive tourist is changing dollars at the official rates.
This PagoFacil/WU office exchanges US$ at the blue rate. Av. Scalabrini Ortiz 2354 x Charcas. Hours: Mon-Fri 10:00am-5:00pm. Friendly staff.
They always have plenty of AR$1,000 and $500 peso notes on hand. Usually no more than a 10-min wait outside. Ideal time is after 12:00 noon. Inside show your US$100 bill(s) at window "ORO" and ask "a cuanto paga" to get a quote.
just walk into any galería that has a storefront that sells gold or jewels and there's a damn good chance they will change at the blue rate for you or can tell you who will.