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ifriction.com comments:

There are several reasons for putting carbon black in a brake lining formula.  If one formula is black, the compounding department people like all mixes to be black - or grey.  It saves on clean out time.  Contamination is not so apparent when all formulas are the same color.  Only about 1% by weight is required for coloring purposes.

Carbon black is a necessary ingredient in most rubber formulations.  For those not familiar with rubber chemistry it doesn't hurt to study the Vanderbilt Rubber Handbook.  It is concise and doesn't need a degree in rubber chemistry to understand.  It contains alot of useful information on the effects of different loading rates in rubber formulations not directly related to friction formulas, but a good guidance.