The Mormon Church restricted involvement and blessings to blacks until June 9, 1978.

On June 9, 1978, President Spencer W. Kimball announced the revelation that all worthy males could hold the priesthood (see Doctrine and Covenants: Official Declaration 2). Following the 1978 priesthood revelation, proselytizing was expanded worldwide to include people of African descent (“Blacks,” Encyclopedia of Mormonism, Macmillan Publishing, 1992).

The Reason Why

The reasons for these restrictions have not been revealed. Church leaders and members have explained them in different ways over time. Although several blacks were ordained to the priesthood in the 1830s, there is no evidence that Joseph Smith authorized new ordinations in the 1840s, and between 1847 and 1852 Church leaders maintained that blacks should be denied the priesthood because of their lineage. According to the book of Abraham (now part of the Pearl of Great Price), the descendants of Cain were to be denied the priesthood of God (Abr. 1:23-26) (“Blacks,” Encyclopedia of Mormonism, Macmillan Publishing, 1992).

23 The land of Egypt being first discovered by a woman, who was the daughter of Ham, and the daughter of Egyptus, which in the Chaldean signifies Egypt, which signifies that which is forbidden;
24 When this woman discovered the land it was under water, who afterward settled her sons in it; and thus, from Ham, sprang that race which preserved the curse in the land.
25 Now the first government of Egypt was established by Pharaoh, the eldest son of Egyptus, the daughter of Ham, and it was after the manner of the government of Ham, which was patriarchal.
26 Pharaoh, being a righteous man, established his kingdom and judged his people wisely and justly all his days, seeking earnestly to imitate that order established by the fathers in the first generations, in the days of the first patriarchal reign, even in the reign of Adam, and also of Noah, his father, who blessed him with the blessings of the earth, and with the blessings of wisdom, but cursed him as pertaining to the Priesthood. (Abraham 1)

We do not trust the Book of Abraham or the Pearl of Great Price and this will demonstrate that The Holy Book of Mormon does not support a skin curse by God upon the Lamanites.

Book of Mormon

6 And the skins of the Lamanites were dark, according to the mark which was set upon their fathers, which was a curse upon them because of their transgression and their rebellion against their brethren, who consisted of Nephi, Jacob, and Joseph, and Sam, who were just and holy men.
7 And their brethren sought to destroy them, therefore they were cursed; and the Lord God set a MARK [fn. 1 Ne.12.23] UPON THEM, yea, upon Laman and Lemuel, and also the sons of Ishmael, and Ishmaelitish women. (Alma 3)

Footnote

23 And it came to pass that I beheld, after they had dwindled in unbelief they became a DARK, and loathsome, and a filthy people, full of idleness and all manner of abominations. (1 Nephi 12)

Misinterpreted

The premise given in Alma 3 is not supported by the footnote of 1 Nephi 12.  The Alma 3 premise says those who rebelled against Nephi were the ones cursed by dark skin, but 1 Nephi 12 refers to his people after they’re destroyed. The word “dark” is 1 of 3 adjectives referring to a spiritual condition, not skin.

19 And while the angel spake these words, I beheld and saw that the seed of my brethren did contend against my seed, according to the word of the angel; and because of the pride of my seed, and the temptations of the devil, I beheld that the seed of my brethren did overpower the people of my seed.
20 And it came to pass that I beheld, and saw the people of the seed of my brethren that they had OVERCOME MY SEED; and they went forth in multitudes upon the face of the land.
21 And I saw them gathered together in multitudes; and I saw wars and rumors of wars among them; and in wars and rumors of wars I saw many generations pass away.
22 And the angel said unto me: Behold these shall dwindle in unbelief.
23 And it came to pass that I beheld, AFTER they had dwindled in unbelief they BECAME a DARK, and LOATHSOME, and a FILTHY people, full of idleness and all manner of abominations. (1 Nephi 12)

Lamanites “by birth” were a pre-existing, dark-skinned people whom Our Lord brought over during the time Joseph reigned in Egypt. Joseph sent forth a remnant at that time knowing after his reign his people would become enslaved. The history of that account is on the Plates of Brass.

Those pre-existing, “Lamanites by birth,” were a divided group before Lehi landed. The seed of Laman and Lemuel intermarried the wicked among them. Their offspring of course reflected this in their skin tone which was a natural result, not a “curse.”

9 And it came to pass that whosoever did mingle his seed with that of the Lamanites did bring the same curse [genes] upon his seed. (Alma 3)