Her Parents had Khoisan blood in them. Remember, i gave parameters (the white parents ancestors back at least to great grandparents should be white also). Anyways, this is about her parents having Khosian blood in them:
IMAGINE you have the same blood as them, and they reject you, just because your mixed heritage is manifest, while theirs remains invisible! Nonetheless, she ended up marrying a black man, and even though her family OBVIOUSLY had African ancestry too, they shunned her and she was forever separated from her family.
Nonetheless, as for the level of African or Khosian ancestry in the Laing family, it had to have been somewhat significant, even though it did not show. While non-white admixture in Afrikaners is 5-7% and therefore negligible, that's just an average. Obviously just as you'll find Afrikaners with less than 5% non-white blood, you'll find some with MORE than 7% admixture. In Sandra Laing's case, her parents happened to be one of the latter. Even in the United States, where non-white blood is more negligible than amongst Afrikaners, there are a some white Americans with above negligible black ancestry. In the case of Sandra Laing's parents, the admixture had to come from BOTH SIDES of the family too. So both had to have had a total of at least 20% Khosian genes, in order for the meiosis to take place. A person who marries a partner with negligible to no non-white ancestry cannot produce a throwback. But if both have enough non-white ancestry on both sides, for meiosis to take place, a throw back is possible! This is the case with Sandra Laing's parents. Negligible non-white ancestry DOES NOT result in throw backs!
I forgot to throw this in there, just in case:
Both Sandra Laing's parents, state that on both sides of the family as far back as their great-grandparents, their family has been white. That still does not rule out the possibility that one of the great-grandparents (or some RECENT ancestor) was "passing", and that a whole bunch of individuals with significant admixture, just happen to enter the genepool!