Black velvet is the most consistently recommended practical choice. Compared with ordinary black cloth or paper, a good-quality dressmaking velvet does a much better job of appearing very dark under direct lighting because its pile reduces visible reflections.

Also look for matte, non-shiny fibers and avoid fabrics with synthetic sheen or specular highlights, which can photograph brighter than they look to the eye. Keep the material smooth and free of folds or wrinkles, since creases can catch light and show up in the image.

Lighting and exposure matter too: even very dark material can turn gray if it’s lit directly or exposed too high. If possible, meter carefully from the black area and place it lower in exposure so it reproduces near black. In some situations, controlling the light or flagging it off the black material may help as much as changing the material itself.

If you want a practical starting point, test a small piece of high-quality black velvet first, especially for sunlight use.