[TMWNN]

A huge skew between the official and black-market exchange rate for hard currency, such as in Argentina (the "dolar azul"), occurs when a) there are capital controls but b) it's still possible to use those dollars/Euro/Swiss franc/whatever. Argentineans are free to use the US dollar to buy things, both inside and outside Argentina.

Right now, it is more or less impossible for any Russian to a) use the dollar to buy anything from outside Russia, and b) travel outside Russia to buy anything in the first place. That greatly reduces the demand for exchanging rubles for dollars, whether via official or unofficial means.