The Blue dollar refers to the grey market exchange rate currently hovering around 950 Argentina Pesos to 1 US Dollar. The official exchange rate is set at about 350 to 1. It is actually quite hard to spend US dollars directly in Arg. However it is common for both foreigners and Argentines to use grey market money changers at the blue rate. Argentines are only allowed to purchase a limited number of US dollars at the official rate each month so if they want to travel or protect the value of their savings from the extreme inflation they need to use the grey market to do so. The Blue dollar exist because the current government artificially pegs the peso at the lower rate.

Not an argentinean, but I don't think that's the right way to look at it.

You can exchange dollars in any country and in many you can directly pay in stores if the storekeeper is flexible. You wouldn't call that a parallel dollar economy.

The only additional point in Argentina is thay they have an official exchange rate that is way way different from the true exchange rate (ie the one that arises from demand and supply). That's why you can exchange dollars through the official channels and get, say, 500 pesos, or you can exchange from an unofficial one and get 1000 pesos