From PLOS Genetics in 2016:
Soheil Baharian,Maxime Barakatt,Christopher R. Gignoux,Suyash Shringarpure,Jacob Errington,William J. Blot,Carlos D. Bustamante,Eimear E. Kenny,Scott M. Williams,Melinda C. Aldrich,Simon Gravel
Published: May 27, 2016
We present a comprehensive assessment of genomic diversity in the African-American population by studying three genotyped cohorts comprising 3,726 African-Americans from across the United States that provide a representative description of the population across all US states and socioeconomic status. An estimated 82.1% of ancestors to African-Americans lived in Africa prior to the advent of transatlantic travel, 16.7% in Europe, and 1.2% in the Americas, with increased African ancestry in the southern United States compared to the North and West.
That’s slightly less white admixture than the usual guesstimate of around 20%. The 3 databases they combined are of older people, with practically nobody born after 1970.
By the way, note how the scientists don’t even bother putting in any kind of Race Does Not Exist weasel words: they just write, “An estimated 82.1% of ancestors to African-Americans lived in Africa prior to the advent of transatlantic travel, 16.7% in Europe, and 1.2% in the Americas …”
Combining demographic models of ancestry and those of relatedness suggests that admixture occurred predominantly in the South prior to the Civil War and that ancestry-biased migration is responsible for regional differences in ancestry. We find that recent migrations also caused a strong increase in genetic relatedness among geographically distant African-Americans. Long-range relatedness among African-Americans and between African-Americans and European-Americans thus track north- and west-bound migration routes followed during the Great Migration of the twentieth century. …
In the HRS, average African ancestry proportion is 83% in the South and lower in the North (80%, bootstrap p = 6 × 10−6) and West (79%, p = 10−4) (Fig 1). Within the SCCS, African ancestry proportion is highest in Florida (89%) and South Carolina (88%) and lowest in Louisiana (75%) with all three significantly different from the mean (Florida p = 0.006, South Carolina p = 4 × 10−4, and Louisiana p < 10−5; bootstrap). The elevated African ancestry proportion in Florida and South Carolina is also observed in the HRS and in the 23andMe study, but Louisiana is more variable across cohorts …
… but the data strongly supports ongoing admixture, predominantly before or around the end of the Civil War. This is consistent with historical accounts of “a marked decline in both interracial sexual coercion and interracial intimacy” at the end of the Civil War. …
See the work of noted historian of antebellum social relations Sir M. Jagger.
The limited role of early 20th century admixture is further supported by the similarity in the inferred single-pulse time to admixture in all HRS census regions (between 5.4 and 6.2 generations ago, S11 Fig) and all cohorts, which is easily explained if most admixture occurred in the South prior to the Great Migration. The similar levels of African ancestry for all age groups within the HRS also support limited European admixture between 1930 and 1960 (Fig 2D). Importantly, more recent admixture is not represented in the SCCS and HRS cohorts; only two participants were born after 1970. …
European ancestry proportions in African-Americans who left the South (16.5%) is elevated compared to individuals who remained in the South (15.3%, bootstrap p = 0.04), confirming that ancestry-biased migrations continued at least to the mid-20th century. These migrants had substantially less European ancestry than African-Americans already established in the North (20.9%) and West (25.0%) (Fig 2E).
This change over time in ancestry-biased migration is consistent with historical accounts that southern African-American migrants to northern cities during the later stages of the Great Migration had darker complexion than North-born African-Americans
So blacks who left the South earlier tended to be whiter: e.g., maybe their dad was white and he funded them to head north.
In general, African-Americans are fairly similar all across the country, with Louisiana being something of an exception due to its Latin social mores. Interestingly, Louisiana blacks have more Native American ancestry as well: e.g., the stereotype of the metis French-speaking fur trapper.
A model with a single pulse of admixture (as considered in) applied to the present data suggests 28.6% Europeans among male contributors, but only 5.2% among female contributors. By contrast, it suggests almost no contribution from Native American males, and 3% from Native American females.