Alma 3

Nephi describes that in his day (what many consider too controversial in our day)that -

"...Inasmuch as they [the Lamanites} will not hearken unto thy words..." (Book of Mormon | 2 Nephi 5:20)

"...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.
24 And because of their cursing which was upon them they did become an idle people, full of mischief and subtlety, and did seek in the wilderness for beasts of prey." (Book of Mormon | 2 Nephi 5:21 - 24)

What is important here is that the "skin of blackness" was not the curse. The curse was -

"...they shall be cut off from the presence of the Lord. And behold, they were cut off from his presence." (Book of Mormon | 2 Nephi 5:20)

The curse was then being cast of was loss of the Spirit of the Lord which caused the described event unless "they shall repent of their inquities". The darkness of the skin was not anything evil or bad, it was amongst those people only for a mark to help seperate those that had made sacred covenants and those that refused to.

After we witness the destruction of wicked Amlici and his followers and many Lamanites, Mormon provides greater insight where the Lord -

"...said to Nephi: Behold, the Lamanites have I cursed, and I will set a mark on them that they and their seed may be separated from thee and thy seed, from this time henceforth and forever, except they repent of their wickedness and turn to me that I may have mercy upon them.
15 And again: I will set a mark upon him that mingleth his seed with thy brethren, that they may be cursed also.
16 And again: I will set a mark upon him that fighteth against thee and thy seed." (Book of Mormon | Alma 3:14 - 16)

Again, the mark was not the curse but only a mark that they would be cursed living with those who refused to accept and covenant with the Lord. Thus they were cursed to lose the promptings of the Spirit having been cast off. Thus speaking of the rebellious of Amlici, using the same logic, Mormon explains -

"...for they also had a mark set upon them; yea, they set the mark upon themselves, yea, even a mark of red upon their foreheads.
14 Thus the word of God is fulfilled..." (Book of Mormon | Alma 3:13 - 14)

The mark of red that the Amlicites did to themselves was their own mark. But mingling with the unrepentent Lamanites, they accepted the curse.