If a couple in Africa with no European ancestry somehow naturally conceived and gave birth to someone who grew up to look just like Sarah Michelle (Gellar) Prinze, would she be white? If a couple in Norway with no recent African ancestry naturally conceived and gave birth to a child who grew up to look just like James Earl Jones, would the child be black? If you think the cases aren't parallel, and one is yes but the other no, why is that?
Presumably in both cases, you're talking about couples who look like the average African or Norwegian - i.e., a black couple in the first case and a white couple in the second.
The cases seem to me to be parallel. One possible difference I can see is that there might be the tendency to suspect in the case of the Sarah Michelle lookalike that we are dealing with a very strange case of albinism, despite the fact that Ms. Prinze does not resemble an albino.
I think there would be reasonable tendency to classify both as the races they *appear* to be, if only as a reaction to the freak-of-nature character of their appearances. That is, in cases when nature seems to bend its rules, we are so excited about the "super"-natural occurrence that we desire to highlight that over any other more natural aspect of the case.