Black market rates are usually more accurate and reflect changes in the economy more.
This is an excerpt from an article posted in July;
The countrys currency recently fell to a record low of 150 Sudanese pounds to the dollar on the black market, compared with 55 at the official rate. The black market rate was 140 pounds to the dollar on Wednesday.
Gee, and I wasn't even aware of the black market here. It seems to be much more inflated and volatile than the official rate and I must have been shielded from it as I've yet to encounter someone selling things at that rate, probably because I don't go shopping much. That's why I was confuaed when I saw articles quoting such large exhange rates like 140 and 150 because they never happened at official rates. This situation is worse than I imagined, and I don't think some people are aware that SDG is severely overralued by the government.
Local items are bargained as hell over here, which you can observe at
https://www.lonelyplanet.com/sudan/a/nar-gr/money-and-costs/355629 but both official and black market exchange rates are so high that prices are still several times higher than they should be.