Just a few recent data points:

Amex:
I had a stay back in mid-February at the Intercontinental Hotel in BA. I paid using my US-based Amex, and on April 4th I was issued a credit titled "Refund US Tourist", which was about 43% of my original payment, which is in line with the official to unofficial exchange rate ratio.

Visa:
Just this past week I made a purchase using a US-based Visa credit card and it posted as the "unofficial" or "blue dollar" rate.

Mastercard:
I also used my US-based Mastercard credit card last week, and although the transaction was authorized at the official exchange rate, in just 2-3 days I received the credit back to my account.

One other bit of info that may or may not have been discussed yet here. I am planning a trip for October to Argentina with the family: BA, Mendoza, Patagonia, etc, and we booked some intra-country flights on Aerolineas Argentinas. Even though I booked the flights from my computer here in the US, I received the favorable "unofficial, blue dollar" rate. I tried both with Visa and Mastercard, and both posted as if I was in the country. Now, I did use an Argentine VPN server, and selected on their website that I was located in Argentina. However, my brother, who did none of that and just booked his flights without thinking about it, also received the blue dollar rate. So evidently you don't even need to use a VPN. As long as you select ARS as your payment currency, and use a US-based credit card, you will receive the favorable rate. So...flights to Mendoza were only $30 each!