This book (found in my local library) is about a white man, a highly-regarded explorer/surveyor in the 1870s and later, who passed as black. He was pure WASP, old New England stock, but in his travels he developed a taste for "ethnic" women. Wandering around Harlem, claiming to be a railroad porter (high-status work for a black man), he met a young woman from the South, married her, and had several children.
Even at that time, it was understood that you couldn't tell who was "black" by their appearance.
Another interesting illustration is Belle DaCosta Greene (real name, Belle Greener), a woman from an old free-black family who was the founding curator of the J.P. Morgan Library. She claimed she had some Portuguese ancestry, but that was bosh.
