What exactly are carbon pencils?
The carbon the pencil producers refer to is to be understood as an abbreviation for 'amorphous carbon' one of the three most common allotropes of Carbon (i.e. stablilized multiatomic structures of atomic carbon) The other well know allotropes are Graphite and Diamond. If you had a diamond pencil you would also have a pencil made of carbon - however a much harder carbon allotrope. Hence it will not produce any line (at least not on paper). See also the wikipedia artikle.

'Carbon pencils' will contain amorphous carbon along with other ingredients to produce a deeper black without the scratchyness of pure charcoal.