For a show that has wasted big chunks of episodes on Betty, there's no reason they couldn't have had an encounter with at least one or two "black" reps from other ad agencies. They did exist, even if they weren't commonplace.
I salute the author for writing something she is passionate about. I read the script and she has the voice of the show down pretty well, even if some of the dialog does run on too long. The problem with it is that Don isn't driving the story. He walks into this Harlem ad agency, says "impress me" and then just sits back. And, unless I missed something, the subplots don't seem to mirror or counterpoint the A story, which is a pretty serious omission for a Mad Men
script. Still, and "A" for effort.