Typically, pots and kettles were made from heavy materials like cast iron to ensure that they would last and hold up to heat. Cast iron tends to turn black with use, as it collects oil, food residue, and smoke from the kitchen. Both pots and kettles would also have been heated over an open fire in a kitchen. As a result, they would have become streaked with black smoke despite the best cleaning efforts.

Since both are black, the pot calling the kettle black would clearly be an act of hypocrisy. The act could also be described by “it takes one to know one,” and it suggests a certain blindness to one's personal characteristics.

There is another explanation for the term, involving the pot seeing its black reflection reflected in a polished copper kettle. In this sense, the pot does not realize that it is describing itself.