"black powder fouling is mostly potassium carbonate which is alkaline."
Uhm... as I understand the reaction, no.
Black powder formulations that don't use sulfur, yes, potassium carbonate is formed by the reduction reaction.
However, formulas containing sulfur (which I believe most still do today) form different reduction byproducts and form little, if any, potassium carbonate.
There's a breakdown on this page...
http://www.musketeer.ch/blackpowder/recipe.html
In the black powder with sulfur you get a significant amount of potassium sulfide in the fouling. Potassium sulfide is EXTREMELY hygroscopic, and when combined with water creates potassium hydrosulfide and potassium hydroxide.
Potassium hydroxide is, of course, basic, but as it has been explained to me there's not enough potassium hydroxide formed by the reaction to fully neutralize the acidic by products that you get from all of the unreacted sulfur (and there's a fair amount) left over, so you end up with a mildly acidic solution.