Take a Q-tip and see if the residue wipes away, it's likely just the powder residue from the blank. Case cracking probably comes from the chamber being slightly oversized for the blank. A note for the slugs, since the blanks are crimped at the tip and the air gun slug rests on it, the blank opens up and damages the bottom skirt of the slug when its fired because there is no space, which would mess with your accuracy. You might be able to increase accuracy of the slug by gluing a printed spacer or a BB between the slug and blank.