Yes—this is mainly an exposure and lighting issue, not a special “detect black background” setting. A phone’s auto exposure tries to average the whole scene, so black paper often gets brightened to gray.

To make the background look black:

• Reduce exposure using negative exposure compensation.
• If available, use center-weighted or spot metering so the phone meters from the coin instead of the whole frame.
• Increase the lighting difference between subject and background: light the coin more strongly and keep less light off the background.
• Raise the coin slightly above the surface if possible, which helps the background receive less light.
• Use a less reflective background material. Black velvet usually photographs much darker than black paper.
• Fine-tune in post-processing if needed.

So the key is: underexpose a bit, meter for the coin, and prevent light from hitting the background.