Hi,
We just have returned from our Uzbekistan trip.
We got 2200 sums for 1 $ at the black market in Bukhara, Samarkanda and Khiva, and this seems to be the "normal" price, but if the $ continues to climb compared to the sums, of course a better rate should be requested.
It's very easy to change money and almost everybody knows where to change, even in restaurants or hotels.
In Bukhara we were offered 2700 sums for 1 Euro and in general there didn't seem to be any problem changing euros, however at hotels and B&Bs it's better to pay with $.
At a Bukhara travel agency we bought flight tickets from Urgench to Tashkent and we paid with sums, because if paying with $ it would have been 38% more expensive!!!
Cheers