Burnt carbon on your casing is normal and not necessarily a bad thing. The type of powder used can be dirtier than others and the chamber on your Henry may not be as tight allowing for carbon deposits to blow back around the case.
It's carbon/powder residue, etched into the case. You oughta see what black powder does to them. It hasn't been hot, brass doesn't turn black when hot and they don't get that hot in the chamber. Did you try 4/0 steel wool to clean it?
THIS is what I think happens in a lot of straight wall chambers; As the case is laying in the chamber and the powder is ignited, the part supported by the chamber wall, that side it is laying on, doesn't have to move...because it's already laying on steel. Before the unsupported side expands to seal off the chamber some small amount of powder fouling blows back on the case, discoloring/etching it. Lots of folks needlessly concern themselves about it. Don't worry about it unless you're just looking for something to worry about. Not all chambers are created equal.