In Lehi’s blessing to Laman and Lemuel he had promised that if they would not listen to Nephi that they would be cut off. They didn’t. Nephi declares that they were cut off according to that divine promise.
What follows is what it means to be cut off. It is a passage that has been read as highly prejudiced and racially charged in many modern readings. The Nephites clearly were prejudiced against the Lamanites, but we must understand the reasons as they understood them, not the meanings that we impose on the text.
Modern readers, including perhaps the majority of the early saints, interpreted a skin of blackness as an indication of a pigmentation change. The context of the sentence itself argues against that. We are quite comfortable with saying that a heart of flint is a metaphor, but somehow the contrast between white and black must be physical. The very fact that neither white nor black describe any actual skin color in the New World should warn us that this is metaphorical.
Of course, the problem is the mention of skin of blackness. Had it been a face of blackness, as imaged in Joel 2:6 or Nahum 2:10, it wouldn’t have been a problem. However, modern racial prejudice has centered on skin color and the word skin triggers those modern issues. They were not ancient issues.
A study of all of the uses of black and white in the Book of Mormon strongly suggests that they should be seen as metaphorical opposites. Even more importantly, there is never a single event in the Book of Mormon were skin color can be seen in the action of the text. In fact, the story told in Alma 55:4–7 relates that Captain Moroni had to search for a Lamanite among his troops and found one. He sent that one with some Nephites to trick the Lamanite guards. If there were one man with black skin, and several with white skin, the deception would have failed immediately. That event underscores that there was no pigmentation change.