I've noticed this with low pressure loads.OP said rounds were dirty.
Low pressure loads often don't push the case against the chamber wall hard enough to make a good gas seal.
Therefore gases will migrate between the case and the chamber and soot up the outside of the case.
The only "brass dust" that I've noticed is from normal cycling of brass cases.I wonder if part of this dirt is brass dust? U reloaders would know about that probably but to me it seems like it may be one and the same thing. Poor manufacturing/dirty rounds with part of the dirt/debris being brass dust. If there any common source of brass dust when reloading that might explain it?
Almost everywhere in the pistol, the brass is in contact with hardened steel, so it's normal for a few tiny brass chips to come off during each cycle.