Here's the thing:
While Mad Men is a series about Don Draper and an ad agency (in that any of the principle characters can be connected to the former or the latter), it's more than willing to follow these characters around. We go into the homes and off-hour lives of many members of SCDP's staff, have plot threads that reveal things about them, and the show's shown a willingness to shake up the supporting cast that it uses in this manner, dismissing some characters even as it elevates others.
There really is no reason why they couldn't
do a story about Don (the secretary) if they wanted to. It doesn't require the series to present SDPC any more tolerantly than it does, but this occurs to me as one particularly obvious point of entry that Mad Men could have an episode that brought in more black characters - Don's home life, her relationships, friends, family, etc.