On the page describing thick film resistors, it is stated that the resistive element is based on carbon. As far as I know, this is not true for any new components that you can buy. My information is that a metal oxide is used. I have seen Ruthenium oxide mentioned for this application.

In the days when I regularly bought leaded resistors for general use, I would always use metal oxide types. Only a cheapskate would use carbon film.

The black colour of the resistor body is nothing to do with a carbon element. The resistive element is protected with a film (glass?), that is coloured black to provide contrast with white text printed on it. I have seen dark green SMD resistors.