Kintsugi basically is a repair using gold or other materials like silver, copper, etc. to cover the break lines or replaced parts. Sometimes these are left black, which is the repair left in its original color, without the metal powder covering. The traditional way of Kintsugi uses 'lacquer' from the lacquer tree which turns pitch black when curing.
This natural lacquer was used as glue in the old times, the metal dust is just for beautifying it. Nowadays even in Japan, where it is common, some repairs are done with special ceramics epoxy because it is simpler, cures faster, and is said to hold longer. Then the metal powder is applied over the break line or repair to make it look more appealing. It seems that even in the old times some items were intentionally broken and then repaired with Kintsugi to obtain a beautiful look.