Howdy,
The amount of residue left behind varies from powder to powder. My Unique powder .44-40 loads smoke more that most .38 Spl BP loads (which doesn't say much for the gamers who shoot them in BP class), but much of that is simply the graphite coating that controls the burn rate of modern powders. It leaves a gray film that wipes off.
What I have noticed is that most reduced .45 Colt loads do not develop enough pressure to swell the brass case and seal the chamber when fired. That results in gas being blown back between the chamber and the case, leaving a red-hot empty and a lot of black soot on the brass. The only cure is to use a heavier bullet, a bulkier powder that better fills the case, like Trailboss, or a faster powder that builds pressure quickly, like Bullseye, AA#2, or Tightgroup.
Bitterroot