Pot calling the kettle black - ethnic slur?

It's clear that "the pot calling the kettle black" refers to criticism that could equally apply to the critic. But why is "black" a criticism in this phrase? I am specifically wondering if this is one of those nasty ethnic slurs that is left over from a time when such things were more commonly spoken, or if some other interpretation might apply. Any ideas or information? Please send an e-mail. Thanks.

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