[from]

You can speculate on nonconvertible currencies with non-deliverable forwards. This is not offered by most retail brokerages.

[gruez]

Who's going to be your counterparty and why would they give you the "official" rate? The whole reason why there's an "official" rate and a "black market" rate is that the government has mandated one rate, but the broader market disagrees and has agreed on a different rate. The two never converge because the government requires you to jump through hoops to get the official rate, so people can't arbitrage the difference.

[from]

Banks I assume? You would get the market rate, not the official rate.

Edit: yes, exactly