Your Kartuschkasten is fine as it is! The Grand Duchy of Hesse-Darmstadt had the luxury of having two different types of officers cartridge boxes for the artillery, dragoons and train: a parade version and a service version (so called interim). The latter was all covered in black leather - probably less susceptible to getting dirty. Yours is the interim one for artillery officers.
Here is a photo where you can see the difference quite clearly.
The bandolier was also different, by the way: one version had a silver or gold cloth covering, while the other was made of leather with either gold or silver buttons.
Very early on, something similar also existed in Bavaria and other German states, but over time it was discontinued there. The Hessians, however, retained it until the First World War.
Best wishes,
GardeUlan