Put it under a microscope - it's a suspension of many particulates from the surrounding areas you rode through. And broken-down oils and old wax and...The reason it's black is due to the refractive index of all these things in a suspension. A good scientist will tell you this is true for all things we perceive as having a specific color. Black is actually considered the absence of color, whereas white is considered all colors at once.

Oh - for some real fun, try viewing it under a longwave UV light.