There is no reason it would have to be loaded with black powder. You can down load with the right smokeless powder to get subsonic velocity. Smaller cartridges are better for this though, as ignition problems start to develop for a mainly empty case, when the cases designed for black power are used.
Having said that, I'm not sure that black powder would pose a problem, except it would foul the suppressor like mad. Black power is not a high-explosive, so I would think it would make for a quieter round to begin with. I know that my old .44 cap and ball is nothing compared to a .44 mag in noise, although it produces a huge fire ball. Although black powder