The term “the pot calling the kettle black” is usually used in the sense of accusing someone of hypocrisy. Despite suggestions that the phrase is racist or nonsensical, the meaning is actually quite obvious when one considers the conditions of a medieval kitchen. 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.

Source: English language and usage