Here's a link to an article on the systematics of the old and new world racers and allied snakes. According to these guys new world racers (like coluber and masticophis) are actually more closely related to Ptyas than they are to old world racers. Not that this is a giant surprise but since Ptyas is also not too far away from the ratsnakes then I guess a black ratsnake and a black racer are not too distantly related. Not close cousins but more like kissin' cousins (so to speak).
Regards
John
Racer systematics

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