Nowadays the expression is usually 'the pot calling kettle dirty bottom'. The original (or so I understood it) was 'the pot calling the kettle black arse'; the cast iron (black in colour) pot, referring to the smoke blackened bottom of the kettle from its suspension over the cooking fire (and it's not always a kettle with a spout either).

I seem to recall that the Ram Jam (which is a pub on the A1 and nothing to do with Robertsons jam with the golly on the label) version of Black Betty actually stated that "she come from Alabam..." which would suggest she was an African American and probably from slave stock. I can see why this particular version of the song might cause offence to some, but at risk of being called racist again, there are some people who really ought to just chill out and let it slide.

LTS