There are a good number of half-black half-white kids at our school, fwiw...
To the PP who asked, at the schools we applied to (Top 3/5), there was no place to naturally or reasonably mention the grandfather's ex job by any stretch of the imagination. It could only really come out by the parents forcing it into an answer to another question about the family (like, "Our family has a long-established love of travel, harkening back to when MOM's father was the ambassador from ___ to the United States."

) I think it would come across as a little pretentious, and given the families around here, it would not really be the be-all-end-all to any admissions team, in my opinion.