Black, as the absence of color, is not an easy proposition for a camera. It is hard for a camera to pick up on black in the same way that the human eye does. If it is not done correctly, the black background can look like awkward blobs that look neither like blank space or a pliable object.
When using a black backgroundm you need to use certain techniques to keep control over the image.
If you are using something like a black theatrical curtain, which is very common in video production, you are going to want to position the lights and the subject just perfectly. If you want it to appear as though a person or object is against a black nothingness, then you want to keep the light away from the background somewhat. Bring the person several feet away from the background while keeping the black overwhelming the frame.
Now, use the three point lighting set up with a light on one side of their face, a fill light on the other and a backlight on the back of his/her head. Do not use a background light at all. Bring the lights closer to the person than normal, especially the backlight. Now, the person will likely be too bright, or “hot,” so you can just reduce the iris until they look appropriate. They will now look correctly lit while the background will look like pure blackness.
If you want the background black curtain to be apparent, then you must approach the situation differently. Keep the subject a normal distance from the background, and keep all lights on them at standard distance.
This time, you are going to use a background light, and make sure it is far off to the side. You want to create sufficient wrinkles and “bunches” in the curtain to add depth. You do not want the curtain to be lit too be perfectly strait because this will end up appearing incredibly awkward.
Black walls are a little bit different because they have the possibility of reflecting light, and you can not create a texture on them. Unless you are using a scrim or gel on the wall, you are going to want it to remain solid black.
You can do this by using the same technique that you did when trying to turn the black curtain into a solid black void, but make sure to bring the subject even farther forward. Make sure that any outside light, such as overhead lights or windows, are completely blackenedd out.
Black backgrounds, for all their faults, are actually some of the easiest to work with. When you are trying to alter a set specifically for your needs, the basic black can always work in your favor. As long as you know how to approach it, you will be fine.