What is the black cartridge?
IT'S A DUMMY.
Once you open the packaging, you'll notice immediately, the Vessel has a BLACK plastic cartridge. Remove the cartridge by turning the cartridge counterclockwise (left) and replace with a pre-filled cartridge of your choice.
The black cartridge is for merchandising purposes only and NOT INTENDED FOR USE. It is not functional and not to be filled with any oils, etc.