No, Black folks definitely nod at each other. But it's not like some covert secret society language. It's just a greeting among "family."
Originally Posted by spiderlashes5000
THIS.

A lot of my white friends would always comment on how I know EVERYBODY, whenever we would walk passed a black person and I would nod.

There are no roots in it, just a thing among "family" like Spiderlashes says.