The New England Genealogical & Historical Register is publishing a paper written by Eben Graves regarding the black or mulatto Sewall families of Maine. It is very interesting and, although absolute proof does not exist for all the family links, what Eben has researched and presented is compelling. [Thanks to Eben for an early glimpse of the paper, in exchange for some basic proofreading...!]
Between census findings, DNA evidence and records of William Dunning Sewall's family found in Illinois, I am pretty well convinced that my son's Sewall lineage is of African descent. I suppose it would not be impossible that George, son of William Dunning Sewall, returned to Maine as a young man, thus linking my son's family back to the Sewalls from England, but there is also a report that an older Sewall member told someone he'd heard there was black blood in the family. There are still one or more DNA tests to be run, but I assume further evidence of African descent will be found.
I thought there would be more physical signs of African-American lineage in the family members I know or have seen pictures of. However, as Eben says, the families have considered themselves white - and been enumerated as such in federal censuses - for generations. Many a black or mulatto Sewall married a white woman, back to the late 1700's.