Black objects, such as black paper, appear black because they absorb a significant portion of the visible light that falls on them. When light interacts with an object, three things can happen: it can be absorbed, transmitted, or reflected.
In the case of black objects, they absorb most of the visible light that strikes them, rather than reflecting or transmitting it. The reason for this is that the material of the object has certain properties that allow it to absorb light across a broad range of wavelengths.
In general, the color of an object is determined by the wavelengths of light that it reflects. For example, an object that appears red reflects predominantly red light while absorbing other wavelengths. Similarly, a blue object reflects mostly blue light.
However, black objects do not reflect any significant amount of visible light across the entire spectrum of colors. Instead, they absorb light across the entire range of visible wavelengths. As a result, very little or no light is reflected back to our eyes, giving the object its black appearance.
It's important to note that "black" is not a particular wavelength of light. Rather, it is the absence of significant reflection in the visible spectrum.