Refraction of light
Black light is basically the lack of light. Since, rainbow is formed in presence of sunlight, so there black color won’t be visible.
What a rainbow or a prism does is break down light into “pure” single wavelengths, and the colors of the rainbow include all the different possible wavelengths of visible light.The prism breaks the light into all the colors that make up white – so you do not see white because white is a combination of colors. Black is an absence of light,something appears BLACK if all light hitting it is absorbed and none is reflected…i.e. the ABSENCE of color or reflected light.