Nephi predicts that righteousness would cause the “scales of darkness” to “fall from their eyes” and, as a related step, result in their becoming a “white and delightsome” people. This prophecy implicitly contradicts Mormon’s text, because these Lamanites have been righteous for quite a while, apparently without becoming “white.” It is perhaps significant that Nephi1 makes no mention of “skin.” Indeed, the “darkness” is associated with the Lamanites’ eyes, not their skin.
Another comment on the Lamanites’ “color” comes from 2 Nephi 5:21: “And he had caused the cursing to come upon them, yea, even a sore cursing, because of their iniquity. For behold, they had hardened their hearts against him, that they had become like unto a flint; wherefore, as they were white, and exceedingly fair and delightsome, that they might not be enticing unto my people the Lord God did cause a skin of blackness to come upon them.”
In this verse, the “skin of blackness” was a social marker, differentiating Nephites and Lamanites so that they would not intermarry. Mormon says that the Lamanites’ whiteness occurred after their political merger with the Nephites. Quite apart from the physiological problem of changing one’s skin color, the change noted here is a political one, not one based upon righteousness. Because the righteous Lamanites had been believers for some time prior to joining with the Nephites, Mormon saw their becoming “white” as a result of their political union with the Nephites rather than their belief.
As I read these statements, I must conclude that “skin became white” and “skin of blackness” are both cultural codes for social distinctions and that black-becoming-white is a metaphor of political change, probably formally acknowledging that the groups may intermarry. It lifts the “curse” in Nephi that ruled Lamanites out as a legitimate sources of spouses. (See commentary accompanying 2 Nephi 5:21 on “skin of blackness.”)