Black is a stunning color for a background. It directs all of our attention to a subject, and it makes that subject extremely prominent and even dynamic. This tokay gecko I photographed during one of my frog and reptile workshops is an example. This obviously isn't a 'natural' looking picture, but it's nevertheless a powerful portrait of this reptile.
Black also looks great behind people, flowers, still life images, cars, leaves, insects, jewelry, and so many other subjects. I prefer black velvet because of the way it absorbs light. You definitely don't want to see the texture or the sheen of the fabric you use for a black background.