Thats partially correct. Not all African Americans have mixed racial blood or relations.
'Among humans, mongrel and mongrelize are derogatory terms for the mixing of "races", known as miscegenation'...Wikipedia
'Miscegenation (Latin miscere "to mix" + genus "kind") is the mixing of different racial groups through marriage, cohabitation, sexual relations, and procreation.[1] The term "miscegenation" has been used since the nineteenth century to refer to interracial marriage and interracial sex,[1] and more generally to the process of racial admixture......Wiki
that true,most african american are not really mixed are i should say a large number are not.