Why does a blue book appear black in red monochromatic light? I found the question on this website but it doesn't provide an explanation.
This occurs because the blue book absorbs all the red, leaving nothing to be reflected, so it appears black.
White light is made up of all the different colors (wavelengths) of visible light. Some of the colors are absorbed by an object and some are reflected. We view reflected light, and we do not view absorbed light. For example, say you're wearing a red t-shirt. The reason it appears red is it reflects more red light than anything else. Objects appear black when they absorb all colors, and objects appear white when they reflect all colors.
This website gives more information about the appearance of color due to reflection. Hope this helps!