When using white paper, dark colors are built up using many layers, light areas need less buildup, and white color is provided by simply leaving the paper blank. Drawings made on black paper are just the opposite. That is, light areas need several layers of color, dark areas need fewer, and black areas are left blank.
There were several reasons I selected black paper for this particular drawing.
I knew I wanted the background to be solid black in order to make the details of the horses features radiate out. Filling in the background with black color on white paper would take many layers of pencil and would not produce as uniform a color as I wanted it to be. (And, building up that great an area sufficiently to achieve a jet black color would take forever and be really boring!)
Also, it is far easier to draw white lines for hair with pencils rather than attempting to create white hair by skipping thin lines of blank paper and coloring in around them. There is a method to burnish, or indent the paper, so that when coloring in later, the white paper stays white where the indented marks were applied; however, this picture had a great many white lines needed and I didn't want to have scratch marks all over the drawing.
I selected a black paper with more of a texture than the white paper I normally use, so when the pencils were applied, they glided on more like pastels, creating a dynamic soft quality. (Pastels tend to have a wider coverage area and have a brighter color.) One great advantage to using colored pencils over pastels, besides the sharpness of details achievable, is that pastels tend to smear and colored pencils stay where you apply them.
The sharp contrast in this illustration, including the white hair next to the solid black background and the white horse standing between two darker ones, helps give the drawing a three dimensional feeling.
Three horses of different colors standing so close to one another was one of the interesting things about this picture that caused me to want to draw it. Each horse has its own unique personality. And the blue eye of the center horse was an outstanding feature which I found mesmerizing.