Article: Colonial records of French and Spanish slave ships and sales, and plantation records in all the former colonies, often have much more information about slaves, from which researchers are reconstructing slave family histories. Genealogists have begun to find plantation records, court records, land deeds and other sources to trace African-American families and individuals before 1870. As slaves were generally forbidden to learn to read and write, black families passed along oral histories, which have had great persistence. Similarly, Native Americans did not generally learn to read and write English, although some did in the nineteenth century. Until 1930, census enumerators used the terms free people of color and mulatto to classify people of apparent mixed race. When those terms were dropped, as a result of the lobbying by the Southern Congressional bloc, the Census Bureau used only the binary classifications of black or white, as was typical in segregated southern states.

Now answer this question: What has helped geneologists researching slaves?
Colonial records of French and Spanish slave ships and sales, and plantation records in all the former colonies