Etymology

There are two interpretations of this phrase, though other sources give only the first interpretation.

In the first interpretation, it refers to the fact that both cast-iron pot's and kettle's bottoms turn equally black when hung over a fire, and thus the pot is accusing the kettle of a fault it shares.

In the second, subtler interpretation, the pot is sooty (being placed on a fire), while the kettle is clean and shiny (being placed on coals only), and hence when the pot accuses the kettle of being black, it is the pot’s own sooty reflection that it sees: the pot accuses the kettle of a fault that only the pot has.

Noun

1. A situation in which somebody comments on or accuses someone else of a fault which the accuser shares.

   I think it's a case of the pot calling the kettle black when she says he is obsessive.