Black colors in insects can be important to the appearance of insects and their visual communication. Many aposematic (warning coloration) insects have the color black combined with red, orange or yellow. Black outlines around a region of bright color can highlight the color and make it more distinct. An indistinct border around a colored region will blend the colors making them less conspicuous. This is part due the visual systems which typically compare the output of neighboring receptors.
The light organs of fireflies are typically yellow and surrounded by black cuticle. The black restricts emission of light to the light organ itself and eliminates scattering through neighboring segments. This makes the organ more distinct. A change from a bright color to black will create the sharpest difference in output. Black surfaces are often found as background to highly reflective surfaces. A reflective surface that is back by backscattering of light, will be less distinct than if it is backed by a black surface that absorbs the unreflected light.