Except for a couple cases like Argentina and Venezuela with currency controls, why would it be different? For those cases, a site like entrusters.com may provide you a ballpark idea of the street rate.
It's an oblique way to come to it, you are essentially doing a swap where you bring something into the country that requires "hard" currency to buy and a person in the country pays you their "soft" currency for it. The rate at which they're willing to pay you vs the item's value in hard currency gives an indication of the current street rate. Not perfect by any means, just the most realistic I've found because someone is actually willing to perform a transaction at that rate. Any other published numbers could be complete BS if they're not transactable.
it is not quite just a couple of cases. there is an offshore rate for the yuan. in columbia there is a black market rate for the peso because the drug cartel have problems turning dollars into pesos..