I've seen ordinary copper traces do the job. But those tarnish and become unreliable. My Commodore computers had keyboards like this. They needed to be cleaned every few years.
The black stuff looks like a dried slurry combining adhesive and carbon/graphite. A homemade version might do the job:
Elmer's is water-soluble, commonly used in school and crafts. White in the bottle. Dries clear, slightly flexible. It turns soft again when exposed to water or heat.
I think the stuff you are looking for is supplied on sheets and heated and pressure punched onto the copper. It is a synthetic rubber compound that is heavily loaded with carbon. I do not know of any supplier but in any case it would need a special application tool. It might be worth your while inquiring at a local PCB manufacturer as applying it to PCBs is a fairly common request. They may be able to spare some if they have any.
Have you considered applying chemical tinning compound, it gives a more durable finish than plain copper and can be applied by dropper exactly where needed.
I think you might be thinking of silicon carbon pills like the ones manufactured by this company (the first one I managed to find). There are a number of companies that seem to also mold and entire silicon keypad with the carbon pill section integrated into the rest of the keypad.