In countries where there is an officially published exchange rate against the dollar, and then there is a parallel rate (street rate, blue rate, black market, whatever it gets called), is Bitrefill able to take advantage of that to increase their margins (and/or reduce the cost for the airtime)?

For example, in Nigeria, the USD/NGN black market rate is above ₦600, while the "official' published rate is ~₦415. Assuming Bitrefill's prices for airtiime, for example, is based on the officially published rate (₦415), that means buying airtime through Bitrefill is much more expensive for a Nigerian than instead first selling the bitcoin for naira (e.g., P2P trade) and then buying airtime with those naira.

Nigeria is one of the more extreme examples, but even in Kenya today there's a published USD/KES rate of ~KSh 117 and then a street rate where a dollar buys KSh 122 or more.

So this is not just an edge case or otherwise is not isolated to just a tiny fraction of Bitrefill's market.