What is the LDS church's stance on the curse of Cain?

And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.

Genesis 4:15

The Bible never says what the "mark" put on Cain was. And I have never seen any LDS-specific revelation about this mark.

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 [the Lamanites].

2 Nephi 5:21

And this was done that their seed might be distinguished from the seed of their brethren, that thereby the Lord God might preserve his people, that they might not mix and believe in incorrect traditions which would prove their destruction.

Alma 3:6

Joseph Fielding Smith elaborated

The dark skin was placed upon the Lamanites so that they could be distinguished from the Nephites and to keep the two peoples from mixing. The dark skin was the sign of the curse [not the curse itself]. The curse was the withdrawal of the Spirit of the Lord.

Answers to Gospel Questions, Vol 3, page 122

This is not the first time God has wanted to keep a group of believers separate.

...thou shalt make no covenant with them, nor shew mercy unto them:

Neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son.

Deuteronomy 7:2-3

Nowadays, there is no such curse, so the dark skin can't be sign of one. Joseph Fielding Smith went on to say

The dark skin of those who have come into the Church is no longer to be considered a sign of the curse. … These converts are delightsome and have the Spirit of the Lord

The Book of Mormon Student Manual covers this topic well in its commentary on 2 Nephi 5:20-25.

As for extending the priesthood to all worthy males in 1978, a couple points should be made:

1. There has never been any record of revelation to restrict the priesthood from blacks. It could have been a divine mandate, or a human custom. But we don't know.

2. It would be odd to say that the tribe of Judah was "cursed" for not possessing the priesthood that the tribe or Levi did. Or that the other Levites were "cursed" for not officiating in the temple as the sons of Aaron did. These responsibilities were limited to certain group of people, based on heritage. God has been expanding these blessings ever since (e.g. the Gospel going from Jews to Gentiles).

Thus, there no canonical, doctrinal evidence for a "curse" on all blacks, now or ever. In light of this absence, it's wrong to teach that there was one.

It was theorized that the mark on Cain was that of a black skin, however, Genesis 4:15 does not specify what the mark was. Moses 7:8 Mentions the people of Canaan, (contemporaries of Enoch) whose land was cursed with much heat and that a blackness came upon them, that they were despised of all men. Moses 7:22 specifies that the seed of Cain were black. Genesis 9:22 Noah cursed the descendants of Canaan, son of Ham, with servitude. Abraham 7:21, 7:24, and 7: 27 mentioned that the Egyptians were of the blood of the Canaanites, had preserved "the curse" in the land, and that Pharaoh came from a lineage by which he could not have the right of the priesthood.

Brigham Young apparently understood these to mean that the black skin of people of African descent meant that they were descended from Cain and inelegible to receive the Priesthood until the rest of Adam's posterity should have the opportunity.

However, this chain of suppositions is weak, which is not to say that it was unequivocally wrong. For instance, it is not clear whether a black skin was indeed the sign of a curse, whether the Canaanites of Enoch's day were descendants of Cain, whether they had anything do with the Canaanites descended from Noah; what the nature of the curse referred to in the Book of Abraham actually was; or whether ancient lineage-based restrictions on the Priesthood still hold. If they did, for some divine purpose never explained, they have since been rescinded.

The Book of Mormon makes it clear (Jacob 3:9) that the dark-skinned Lamanites were not to be reviled for the blackness of their skins and that it was no reliable indicator of their standing with God, and furthermore that even those whose lineage had been cursed could escape it (Alma 23:18) by conversion to Christ. It always had more to do with the false traditions of their fathers than the color of their skins.

To give the church the most benefit of the doubt, I have to point out that 2 Nephi 5:21-23 doesn't refer to Cain and his descendants but rather Lamanites. In this case dark skin and being cursed are the same thing, but you could argue this is not necessarily the case with Cain.

This is what the KJV (the Bible text accepted by the LDS Church) says about the mark put on Cain:

15 And the Lord said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the Lord set a mark upon Cain, lest any finding him should kill him.

Genesis 4:15

According to LDS scripture, the descendants of Cain were actually black, and it is implied through the footnotes that this is the same as the mark. However, it's not obvious if this is technically a curse.

There is also no canon scriptural source that clarifies whether Mormons consider all modern day people of African descent to be descendants of Cain. However, some early church leaders have claimed that to be the case; for example, Brigham Young:

What is that mark? you will see it on the countenance of every African you ever did see upon the face of the earth, or ever will see.

In more recent times church leaders either don't comment on the matter or claim ignorance. The official church essay on Race and the Priesthood for example mentions Cain and the curse several times but does not clarify what the church's previous opinion on it actually was.

In conclusion: if you allow for the "mark" of black skin being different than a "curse," you could say the church never believed black people were cursed. However this is unlikely given the following quote from Brigham Young in the same speech that the official church essay quotes:

Now then in the kingdom of God on the earth, a man who has has the Affrican blood in him cannot hold one jot nor tittle of preisthood

So, if you consider the "mark" and the "curse" to be one and the same, then it appears the church changed its stance in 1978 or since then. There has been no official acknowledgment of this change to my knowledge.