The story
The Advanced Space Vision System images objects with cooperative targets and uses the known positions of the targets to triangulate their exact relative positions in real time. The targets are composed of thin films of silicon dioxide layered with inconel to form an inconel interference stack. A stack like this has nearly no reflectivity in the Electromagnetic spectrum. The result is a black color that appears even blacker than the flattest black paint. In photos the disks look like small black dots, and a minimum of three are needed, so they are quite unobtrusive on most payloads.
From Wikipedia’s article on Advanced Space Vision System, shared under CC BY-SA 4.0.3