Can you? I've got to imagine that transacting in a runaway inflationary currency is going to get you bad rates no matter what.

If you want to buy crypto using rubles, Lebanese pounds, Venezuelan bolivars or anything else, you need a counterparty who will accept that currency as payment. And if your currency is losing a lot of its value every day, with no end in sight, your counterparty will not give you anything approximating the official exchange rate, even on an open market, because accepting payment in that currency immediately exposes them to that currency's inflation.

it's pretty much the same. Black market and crypto market are arbitraged between them so the price of one isn't too different from the other

That doesn't make sense. The black market (being illegal) would have significantly higher risks and transaction costs, so even with arbitrage the prices could still be significantly different.

Ok but without crypto would black market rates rise? I gather that was the point.

Oh that's a different question IMO. Crypto is making it easier to bring dollars from abroad so I get it does lower the black market cash dollar's prices in that sense. Big international crypto exchanges like binance aren't yet regulated here so technically the crypto world IS part of the black market anyway