[cardaholic]

I would not be surprised if some paper advertising had been created using the pictures of Negro League stars for businesses that served black customers, and I would expect this to be most likely in the formally segregated South (I'm guessing restaurants as the most probable advertisers). These might well have been limited to ephemera that most of us would not now consider as cards - newspaper ads, etc. Stand-alone advertising flyers would have been reasonable, and those of us with broader definitions of cards might deem those to be cards.
It certainly would seem worthwhile to investigate - Birmingham, Memphis, Nashville, Atlanta, and Chattanooga black newspapers would be the most logical starting point. New York black newspapers would be sensible too. Of course, if you were to find in a library archive an ad featuring a Negro League star as a pitchman for some local restaurant, then you'd have to try to find some other copy you could purchase (unless you're Barry Halper, in which case you'd pay someone to steal it). [Granted, you could find a nice niche market in reprints of a newspaper ad if you think you can do it without fear of a copyright lawsuit.]