I believe the “Black Dutch” term is based in reality, though the meaning may mean different things to different people. I’ve also heard the term Melungeon to describe some of mixed blood and met such people in West Virginia in the 60s. These kinds of mixed blood people were discriminated against by both blacks and whites. The term Creole is another mixed blood term.
My family is from NC, KY, AR, OK on one side and European on the other. My father always said he was “Black Dutch” when we asked what he was. I have done research and from what I could find in collaboration with a cousin who heard the same thing, we think it is a term which originated from them having mixed blood and a fear of society. I cannot yet prove it but current evidence leads me to believe it is from having Black ancestry. I do not believe it is from Indian ancestry, though my great great grandfather was supposedly a Choctaw chief.
It doesn’t matter to me if my ancestors are of mixed blood as their unique blending are why I live and exist. I find it interesting and am only interested in finding those connections for family history reasons. I do know there will be some who will have a hard time accepting they may have mixed blood.
One last item which may be of interest, I have a niece, very caucasian with European father, mother and grandparents and she carries the gene for sickle cell anemia. Tells me we may well have Black ancestry somewhere, even if it is very far back.