Coon Pickle

Watermelon - The joke is that white people apparently eat a lot of pickles, so since black people stereotypically eat a lot of watermelon, they're black people's pickles.

'Ey Cletus! Ye' wantin' te' buy sum' coon pickles?
Da' hell is a coon pickle, Betty-Sue?
Dem' here watermelons, of course! Dem' black peoples eat'em up like pickles.