How are you defining a "white" person? Simply as someone who manifests with white skin (notwithstanding albinoism)? If, for example, you have white skin - and even all the other physical characteristics of a "white" person - but are yourself of mixed black/white parentage, you could marry a white person but still give birth to a black child. It's all about the genes, and which are dominant in the child. And the dominancy of some genes might skip a generation. It's not common, for sure, but it is possible. Your quoted comment above is a reasonable rule of thumb, I think, but not absolute.