These verses parallel the description Nephi gave in 2 Nephi 5:20–23 concerning the curse on the Lamanites. In 2 Nephi 5:21 a “skin of blackness” is mentioned during the description of the curse. In Alma 3:6, there is a dark skin which was a mark of the curse. Importantly, it is not the curse itself. In the discussion of 2 Nephi 5:21, it was noted that there is no evidence that the dark skin or the “skin of blackness” ever created a visible difference among the peoples. It was a metaphorical condition that perhaps people believed that they could perceive, but which was based on unrighteousness rather than pigment.
It is important, therefore, to remember that when Mormon is describing the visual differences, he mentioned the Lamanite shorn heads. Had all Nephite, and the Amlicites who were recently Nephites themselves, had a dramatically different skin color, there would have been no reason to note the shaved head as a distinguishing mark.
In both 2 Nephi 5:23 and in Alma 3:9, it is a curse that would come from intermarriage, but it is a cursing that is dependent upon righteousness, not genetics. Note that in 2 Nephi 5:22–23 it says: “And thus saith the Lord God: I will cause that they shall be loathsome unto thy people, save they shall repent of their iniquities. And cursed shall be the seed of him that mixeth with their seed; for they shall be cursed even with the same cursing.”
The parallel in verse 8 above that the prohibition that was part of the curse was instituted: “that thereby the Lord God might preserve his people, that they might not mix and believe in incorrect traditions which would prove their destruction.”