Although it may seem as though black could only be one color, there are actually multiple different “shades” of black that exist. The color that we call black is actually what is known as
pure black.  When we see things that aren’t quite pure black, we use the inaccurate term “shade” of black. The term “shade” for these off-black colors is inaccurate because according to color theory, a shade of a color is any pure color mixed with black to decrease the brightness of the color you choose.  A more accurate term for these “shades” of black is calling them off-black.  Off-black colors differ only slightly from pure black while still keeping the low lightness and the low relative luminance that black has.  Off-black colors have their own hue and saturation.  Some examples of off-black colors are onyx, black olive, charcoal, and jet.

The reason we see these black tones differently is due in part to the photons and the reflection of light that colors have.  Black is when there are no photons at all reaching your eye.  A black surface is one that absorbs all of the photons so that none of them can hit your eye. The reason we can see different variations on the color black is because of the saturation of different colors that are making up the black.  For instance, in the color Onyx, the RGB levels are 53,56, and 57 respectively, whereas in pure black the RGB levels are 0,0,0.