Another explanation to couple with the carbon replies is how various matter reflects/absorbs frequencies of light differently to appear a certain colour. Take the life cycle of leaves for example. In the summer, leaves are at it's healthiest and have tightly formed cell structures that reflect the most light in the green section of the spectrum. As autumn approaches, temperature drops and the overall health of leaves decline as well. As this happens, the cell structures of leaves becomes "looser" (for lack of a better term) and less capable of reflecting green light waves and instead absorb that section of the spectrum, leading to the yellow/red end of the spectrum to be the highest reflected portion. This process continues until leaves fall off the tree, to reach its unhealthiest/dying state where the cell structures no longer reflect bright green/yellow/red sections of the spectrum and reside to the a static in-between portion of the spectrum - brown. Applying the knowledge of cell structures and correlating wavelength of light being absorbed/reflected, my best guess would be that once something is burned - the altered cell structure becomes able to absorb all wavelengths of the visible colour spectrum, reflecting no visible colour at all - and appearing black.