Checking around the internet I have found one example of the slave medallion in white-on-blue, this one at the American Philosophical Society in Philadelphia.
I referred before to the image of the supplicant black man being unpopular with some. It grew to be abhorrent because the image took on a life of its own and was published far and wide. It also inspired similar images in art, and for quite a time reinforced an image of black inferiority.
ADDED 19/07/09: Janson Hews of the Powerhouse Museum has pointed out an excellent example of the slave medallion with commentary at the V&A museum.
ADDED 31/07/09: The page from the online Wedgwood Museum about the Slave Medallion is here.