The reiteration of prophecy in verse 20 contains only two elements; that the Lamanites would be cut off from the presence of the Lord, and that they were cut off. In verse 21 Nephi continues with a "cursing." Because this verse ends in the controversial "skin of blackness," it is of worth to examine the verse carefully.
The first sentence indicates that the Lamanites were cursed with "even a sore cursing." The cause of this cursing is clearly stated as their iniquity. The second sentence is a compound sentence, and contains two ideas. The first is the concept that the Lamanites had hardened their hearts against the Lord, so hard that they were "like unto flint."
It is the second part of the sentence that the most interesting. Inside of the phrase on skin color there are two overt concepts. The first clear concept is that the Lamanites were, at the time of separation, " white, and exceedingly fair and delightsome." Of course this is only to be expected, because they were siblings to Nephi and Sam and the rest of the Nephites. Certainly Nephi would not be expected to say of them anything so petty as that they were ugly, and we are "fair and delightsome." Nevertheless, Nephi indicates that there was a change, that that they now have a "skin of blackness." What is going on?
If we do some arbitrary re-cutting of the text into verses and move the sentence on the cursing to a different verse so that it is not as clearly seen in this context, the final verses have a very consistent version of the skin color. There is a change in skin color, and the change occurs under the direction of the Lord for a particular reason. The reason is "that they might not be enticing unto my people." This reason and function for the skin color is reiterated in verse 22. The function is to provide a visual separation between the Nephites and the Lamanites. Just as ancient Israel was under proscription from marrying outside the ethnic group, the New World Nephites are also being set up as a separate unit, prohibited from intermarriage (v. 23).
The text does not clearly indicate what that curse was, and the reason for the skin color change is given after it is announced, suggesting that the skin color might not be the curse, as that change required its own explanation. However, it has become customary to presume the connection between the curse and the skin color. In the context of Nephi's cultural predispositions, that could easily have been his perception.
The next problem is the phraseology itself. Until the importation of African slaves, a true "skin of blackness" was unknown (or at least quite rare) in Mesoamerica. When Cortez arrives with blacks in his company, the natives were even more taken with the novel black skin color than with the "white" of the Spaniards. With meaning did "skin of blackness" have for Nephi?
Nibley further expounds on the cultural meanings of the black/white problem in the Book of Mormon:
John L. Sorenson further examines the context of this "skin of blackness:"
The immediately salient points are that this is absolutely a recognition of prejudice, and secondly a continuation of a long tradition that tends to exaggerate the "terrible" nature of ones enemies. It is a common feature of small group dynamics that where enmity exists, there will also be prejudice, and where prejudice, exaggeration. All of this is happening between the Nephites and the Lamanites.
Because we can see the Nephites as possessing a prejudice typical of their age, does that mean that we impute prejudice to God? Of course not. God's "hand" in this matter was to mark the Lamanites as separate. The prejudices came from the Nephites themselves. But where did the skin color come from? Did the Lamanite simply wake up one morning and see everyone around them with a markedly different skin color (can you imagine the shock)?
The best explanation is a combination of the lifestyle and the intermarriage with existing local populations. Remembering that the Lamanites would have remained along the coast while the Nephites went inland and upland, the Lamanites chose to wear fewer cloths (due to the hotter and more humid weather - not simply their indolence, as suggested in the Nephite record). Such conditions alone would darken the skin. Intermarriage would have taken longer, and the likelihood that the Nephites also intermarried with other populations suggests that the real difference was one of darkening due to sun exposure.
As John L. Sorenson notes:
What we have in this verse is an adequate explanation of the consequences of the Lamanite lifestyle choice, along with a culture-bound prejudice system, expressed in terms common for Nephi.