I bought a box to try out in Arkansas. They are VERY dirty. A lot of it has to do with the buffer they are using and the powder from what I could tell. The wind was wrong on one shot and I had all sorts of shit blow back into my face and eyes. Had to stop shooting to clean out me eyes.
After one box I looked at the internals of my gun (SBEII) and it had a ton of crap in it. I normally shoot Kent Fasteel and it takes me a lot more than one box to get that much gunk in it that I got from shooting one box of black cloud, IMHO.