This is the black, watery residue of sponging out a fired cannon. The sponge/rammer (a person) dips the sponge into a bucket (usually filled with disgusting-looking black water) and then spins it between the hands to get most of the water off and leave only a damp rather than wet sponge (otherwise the gun, once sponged, may not fire again).
After spinning the rammer to remove excess "water," the sponger will usually find several splatters on his clothing. These splatters are politely called cannon splooge or gun splooge. See also musket splooge.
Most sponge/rammers will step away a bit before spinning the rammer, or will otherwise be subject to a stream of invective from his gun-mates that is worse than any gun splooge.