For what it is worth, I found that CleanShot, American Pioneer, Shockey's Gold (or whatever else it's called now) is the cleanest substitute on the market. It cleans easily and is definitely less corrosive in normal use. It also tends to absorb moisture from the air at an alarming rate, so much care has to be given it in that regard.
As far as the Black Mag 3 goes - I got a lb of it and found it to be complete junk. It was way underpowered, even compared to CleanShot (which is about 15% weaker than Pyro RS or true black powder). It also proved to be very inconsistent in my Knight -bullets that cut dime-sized groups using 777, would not rerspond well to any charge of Black Mag.
For me, 777 works best. It's powerful, cleaner than Pyro or black, loads consistently, and doesn't seem to go bad like the CleanShot stuff. Some have had issues with the crud build up caused by 777, but I've found that by keeping your power loads down (100gr being my largest load) I've not had any issues out of cap-igniting Knight.