When an object appears blue when seen in white light, it means that the object selectively absorbs most colors of the visible spectrum and reflects primarily blue light. In this case, if you illuminate the object with red light, which has a longer wavelength than blue light, the object will still absorb most of the colors except red. Since the object does not reflect red light, it will not appear red; instead, it will appear dark or black. This is because red light does not contain the blue light that the object can reflect to appear blue.