At the start of the civil war there were 4.4m blacks in the U.S. Your challenge is to reduce this number to less than 1m by the year 1900 without mass genocide. And if the back population were reduced how would that effect the U.S.
Simplest thing - don't have miscegeneation or segregation laws at all, no racial barriers on marriage or relationships. Develop a strict and narrow definition of black.
Encourage interracial marriage, and legally classify the children of such marriages as white.
Without segregation, ostracization, miscegenation laws and other barriers, a lot of the black population will simply merge with the white population. Leaving a white population and a remnant pure black population.
Given ingrained racism.... good luck.