Carbon Brakes
Unable to give you a reference on this.
In general, carbon brakes contain a significant amount of carbon fibers and Silicon Carbide (SiC) , a material which is also used in abrasives. SiC in carbon brakes is formed during the manufacturing process, where a porous carbon fiber and matrix preform is infiltrated with liquid silicon and thereafter "baked" at very high temperatures to form the SiC. The black "dust" you see is a composition of this SiC and other components of the brake (micron size carbon fiber fragments etc.).
Carcinogenic effects of SiC dusts are under investigation with workers in SiC plants. For carbon brakes, I don't know if there is a similar study available.
IMHO, the risks related to carbon brakes are minor since the carbon particle emissions (submicron to angstrom range) of the engines have a much more severe effect on the health of ground personnel.
F.y.i. beryllium brake disks have been used on military aircraft (C-5 Galaxy, F-14 Tomcat) due to their superior energy capacity, yet its dust (beryllium oxide) is severely carcinogenic.