El dólar blue hoy se vendía hasta 142 pesos en las cuevas del Conurbano bonaerense, donde los arbolitos y cueveros se hacían el día ante la sostenida demanda del billete en el mercado ilegal.

This I read in a publication. Let me first loosely translate it as if I were Google:

The blue dollar today was being sold for 142 pesos in caves of the urban core, where little trees and cave dwellers were having their day with the sustained  demand for paper cash in the illegal market.

A better translation would be:

The black market dollar was being sold for 142 pesos in the Buenos Aires city centre by the motionless men at city corners (thus little trees) who dealt from offices called caves. They were having a heyday because of the high demand for US Dollars.

Since I can remember, (1965) while as a conscript of the Argentine Navy I would every once in while change some of my saved US dollars with arbolitos who would stand at corners of Calle Corrientes. Most now (and a recent development of arbolitas of the female kind) are on the busy pedestrian mall of Calle Florida. When we were last in Buenos Aires in September 2019 we would change our $50 US Dollars for pesos (nowbody would arrest you and you are never cheated) for 65 pesos. Note that these blue dollars are now at 142.

Some say that when Diego Maradona was asked about the black market dollar that he answered that he did not care if they were black, green or blue.

It was in 1966 when I passed by a beautiful shoe store called López Taibo on Calle Corrientes. I spotted  a beautiful pair of brown leather boots. I entered the store in my sailor uniform (as sailors we were paid the equivalent of one US Dollar a month since the pay standard had not been changed since 1902 or thereabouts.). I asked a very serious man to show me the pair. He instantly said,  “Are you sure you are in the right store?,” and he stared at my uniform. From a pocket I removed a 100 US Dollar bill. I left with the boots and lots of change.

Sometime in the late 80s or perhaps in another trip in the 90s I went back to Argentina. Lopez Taibo was still on Calle Corrientes but in a much smaller space. My previously purchased boots no longer fit me. I wanted a new pair. I went in the story and the same man was still there. He asked for my name and came back with my foot measurements. I left the store with a new pair of boots that were still a bargain.