I don’t think Hymie is specifically African American, nor is it particularly regional.
I too considered Get Smart when I was writing up the entry. I concluded that since the derogatory use didn’t appear for another fifteen years Brooks was just making a joke about a Jewish robot, and that Hymie the Robot was hypocoristic rather than derogatory. But given the 1973 cite in HDAS, it may be that Brooks was doing a bit of reclaiming of the name which was edging into derogatory use.