Win 231 is a dirty burning powder in most loads. So is Bull's Eye I still use Bull's Eye for most of my handgun loads. It will clean off. Most factory .38 spcl loads are dirty as well.
If you want soft shooting 158 LSWC's that are a clean load. Try using about 4.0 grains of Trail Boss. Very clean burning powder. The black stuff you see after shooting with it is a result of carbon from the buring lube.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.