(This is the beginning of a good idea. Maybe I will back it up and write a famous article someday. Here is my first cut.)
Part of the hidden work of protecting their families that dads do is to move their families to safer neighborhoods. Couple-headed families usually have more than twice the income of single-mom families, and can afford better neighborhoods. Fathers provide direct protection for their own households. Fathers also usually contribute to everyone's safety in their own neighborhoods in a way that families in fatherless neighborhoods rarely do.
Douglas Massey and Nancy Denton won the big sociology prize a few years ago for American Apartheid: Segregation and the Making of the Underclass. They argued that American cities were resegregating, especially by separating African-Americans from other Americans in neighborhoods that were increasingly black. Massey and Denton attribute this to racism.
One of the ways that African-American families are distinctive from other racial-ethnic groups in this country is that black children are much less likely to grow up with their married father in their home.
Putting these two ideas together:
Fatherless families tend to end up concentrated in the least expensive neighborhoods; and
Fatherless neighborhoods tend to become the most violent; and
African-American families are the most likely to be fatherless and live in fatherless neighborhoods; so
One of the main underlying factors in the increased racial concentration of African-Americans is really an increased fatherless concentration of African-Americans.