Alma 3:6-10 is a conceptual unit that shows a dramatic affinity to 2 Nephi 5:20-23. We shall examine the parallels closely.
2 Ne. 5:20 Wherefore, the word of the Lord was fulfilled which he spake unto me, saying that: Inasmuch as they will not hearken unto thy words they shall be cut off from the presence of the Lord. And behold, they were cut off from his presence.
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.
22 And thus saith the Lord God: I will cause that they shall be loathsome unto thy people, save they shall repent of their iniquities.
23 And cursed shall be the seed of him that mixeth with their seed; for they shall be cursed even with the same cursing. And the Lord spake it, and it was done.
While the language is not absolutely identical, the set of themes is clearly suggestive that the text in Alma has a dependence upon 2 Nephi. Alma 3:6 introduces the main elements of the cursing of the Lamanites. They have a dark skin, and that cursing comes because of their rebellion against Nephi. The 2 Nephi 5 text begins with this very idea that the rebellion against Nephi is the root cause for the eventual cursing of the Lamanites. 2 Nephi 5:20-21 explains that there is more than a spatial separation between the Lamanites and the Nephites – there is a perceptual barrier.