These backgrounds usually look much cooler than white since you can create more dramatic light. But they are also much trickier to deal with. Black mirror surface normally stays black no matter how much light you throw at it but it gets lighter when you can see reflection of the light source in it. That’s why umbrellas are not normally used for this kind of photography, you will need good rectangular softbox with not-wrinkled screen.
For this Bugatti frame I positioned softbox horizontally angled approximately 45 degrees relative to lens axis to create visible transition line from black to grey.
Here I used black velvet to remove any reflections from background. A lot of negative space is left for additional text to be put in.
This pictures was taken on black mirror with black foam board at the back. One light with red gel was positioned below the table and aimed high to light back panel and reflect in the mirror.
Jewelry is probably one of the hardest things to photograph. If you don’t light stones properly they would look completely dead and this Tag Heuer L-type frame coincidentally had a whole set of small rubies on side. This shield of rubies is main point of interest of this frame, and I used purple gel as background for this shot to compliment gemstones color. I lit frame from above with the softbox, and I used another light with 7” reflector and 10° grid to bring rubies to life.