Why did "Lift Every Voice and Sing" become the "Black National Anthem"?

Attending an African American church service recently, I was introduced to the hymn [Lift Every Voice and Sing](http://en.wikipedia.org/wiki/Lift_Every_Voice_and_Sing) . One of my colleagues said this was the "Black National Anthem," uniquely Christian and a great solace in the fight for civil rights. What is it that caused this song to be so closely identified with the Civil Rights Movement, and is its theology unique to a particular brand of Christianity? Along these lines, is there a scriptural basis for this hymn?