Here is an interesting case. Sally Miller
http://en.wikipedia.org/wiki/Sally_Miller was freed when Louisiana judges ruled she was White. Allegation was that an indenture servant German girl was orphaned during her indentured period and was never freed and her master claimed she was black. These were Southern Judges who would have seen many mixed race individuals and would not have been easily fooled. These Judges stood on honor and ruled what they truly believe which cost them their jobs.
Basically, there undoubtedly were masters who would take a White child, an orphan or one who was spirited away, and claim that the child is a light skinned black since slaves were worth a fortune. Also, did all master free their White slaves? What about an Irish slave breed by her White master and the child in turn breed by a White master and so on, you could have a few pure White slave left several generations later. It would not surprise me if a majority of those "White" slaves weren't in fact White.
I have seen blacks that blacks don't believe are black and they think that can pass for White, but White can easily tell they aren't White. Let's face it only a very tiny of blacks can pass and many that they believe can can't.
There is a panic whenever White slaves are talked about so the egalitarians try to claim that there were no White slaves and what were called White slaves were really light skinned blacks or indenture servant. Just another dirty secret of history that the "antiracist" want to buried.
As for the Melungeon, there are so many different groups who get that label and the results of testing are all over the place some come out as pure White, some as White Indian mixed and some as black White and/or Indian mixed. The black/White mixed is what they talk about because that is politically correct today but probably a very tiny number and who know if the scientist got the DNA right it could be European DNA that ended up in Africa especially if it is found in both areas and only a small part of Africa. Don't forget that Whites were once in Northern Africa and they could have let DNA traces in sub-Saharan Africa especially if it is in the Northern Part. In time, what we are told about DNA may not be what is discovered later