HighUintas, I have never used it, but if it has enough contamination to be concerned about, I would not go through the headache of trying to clean it. That said, KNO3 is water soluble. You could dissolve it, pour it off the 'black flecks', or skim them off the solution, depending if they sink, or float. Then, if they are not water soluble, haha, you can boil it dry and grind it up again. If they are water soluble, please forget everything I just wrote.
Here's a link to Seed Ranch, from Florida, to their KNO3. I have bought 10 pounds from them, twice. The last I bought was about $2.40 per pound, including shipping, buying 10 pounds, a couple of years ago. I see when I went to get you the link, the price is that much, before shipping now. They used to beat Duda on price, but things may be different, now. I will vouch for their 99.8% pure powder. It is clean fluff powder.
I have not used a bit of it, yet, but I bought a 50# bag, of "98.9 % pure" prilled KNO3 from my local farm fertilizer dealer, for less than $20. I have a farm, and have known them for years, and I actually got a #2 scoop shovel full, for ten bucks and an offer to buy beer. It looks great, just not puffy powder. It's little bb's. I figure, for the price, and availability, I'll figure out how to powder the bb's. By the way, if your wife has roses, they'll love your KNO3. Which ever way you go, good luck and here's the Seed Ranch link, if you're interested.
https://www.seedranch.com/Potassium-Nitrate-s/2137.htm