What is the frequency of the color black? Our eyes don't see light; they detect vibrations in the 400-800 THz range that we call color. Since our eyes can detect the color we call black, what is its frequency?  
 A: Black is not a color, it is a shade.
In physics, we call something "black" when it does not reflect any of the incident light. However, all black bodies radiate. The frequency of that radiation, the black body spectrum, is a function of the temperature of the object and follows Planck's Law:
$$B(\lambda, T) = \frac{2hc^2}{\lambda^5}\frac{1}{e^{\frac{hc}{\lambda k_B T}}-1}$$
Plotting this for a well known "black" body (the sun) you get the following (normalized):

We recognize that as "white", but really it is black.
Even white objects only appear white when we illuminate them with white light - if you shine red light on a white object, the object doesn't change color, but you perceive it as "red".
So there are three things going on here:


*

*When you look at an object, what you see is the interplay between the color of the incident light, and the surface reflectivity

*"black" means "does not reflect light"

*black bodies have a color corresponding to their temperature: colder = redder

