It is also important that the object you are shooting is not resting directly on the black background. I use black velvet, which reflects relatively little light, but if a small insect sits directly on top of it then the fabric’s texture will be plainly visible. Thus, I place any subjects that can be shot in a vertical position (a plant, an insect on a branch) at least 20-30 cm away from the background, usually by holding it in a “helping hand” clamp. If the animal needs to sit horizontally, or if I want to shoot it from above I place it on a piece of glass or clear plastic, held horizontally about 20 cm above the black velvet. In the latter case it is very important that the lights are positioned in such a way that reflections on the glass are minimized (unless a visible reflection is something that you want); proper positioning of the lights can only be found by trial and error, and the glass should be as clean as possible because any smudges or specks of dirt will show against the background.
Here is the setup I use for my black background photography.
A. Black velvet – it is important that the velvet is draped uniformly, without folds or wrinkles; they will reflect light and show as lighter patches in your photo.
B. A piece of thin glass or clear plastic – I use it to move the subject away from the background; if the background is too close its texture will show in the photo.
C. Two diffused side lights – I use Canon 580EX/580EXII flashes in Photoflex LiteDome XS Softboxes; these softboxes are very light, and can be easily folded and stuffed into a camera bag for use in the field.
D. Backlight – another Canon 580EXII, placed high above the black stage, diffused with a large sheet of white paper; I often turn this light off and only use the two side lights.