I will post the link from the Census article on this below but first you need to know it is wrong. Black people have been undercounted since the beginning of the republic and it has only been close since the 1990s. It is still off for Black males by about at least 5% I believe. They know it is inaccurate because they use birth and death records by race to compare expected and actual numbers for Whites and Blacks.
Historical Census Statistics On Population Totals By Race, 1790 to 1990, and By Hispanic Origin, 1970 to 1990, For Large Cities And Other Urban Places In The United States
In the 1970s some demographers at Princeton and Upenn used advanced techniques to extrapolate the Black population. I will put their numbers below for certain years in the % of population that was Black. Pretty much except for the bump around 1960 we have been losing share for almost 200 years. I think that is
@4d 6f 6e 65 79 point. Also
@Booksnrain
Approx. Black pop in thousands and %:
1820 1,800, 19%
1830 2,300, 18%
1840 3,000, 17%
1850 3,700, 16%
1860 4,500, 14%
1870 N/A
1880 7,200, 14%
1890 8,500, 13%
1900 9,900, 13%
1910 11,100, 12%
1920 12,300, 11%
1930 13,600, 11%
1940 14,700, 11%
1950 16,700, 11%
1960 20,700, 12%
1970 24,400, 12%
From:
A Statistical Reconstruction of the Black Population of the United States 1880-1970: Estimates
of True Numbers by Age and Sex, Birth Rates, and Total FertilityAuthor(s): Ansley J. Coale and Norfleet W. Rives, Jr.Source: Population Index, Vol. 39, No. 1 (Jan., 1973), pp. 3-36
Growth of the Black Population in ante bellum America, 1820-1860
Author(s): Jack E. Eblen
Source: Population Studies, Vol. 26, No. 2 (Jul., 1972), pp. 273-289