No good counter-argument has been published by LDS scholars.
2 Nephi 5:21 says God cursed the Lamanites with "a skin of blackness," so they would not be "enticing" to the Nephites. Alma 3:6-9 marks their descendants.
3 Nephi 2:14-15 has converted Lamanites becoming white. Jacob 3:8-9 and Mormon 5:15 belong with these.
Acts 17:26 says God made all nations "of one blood." Acts 10:34-35 says he shows no partiality. 1 Samuel 16:7 says he looks on the heart.
Galatians 3:28, John 7:24 and Revelation 7:9 point the same way.
In 1981, "white and delightsome" became "pure and delightsome" at 2 Nephi 30:6. In 2010, racial wording came out of the chapter headings of 2 Nephi 5 and Mormon 5.
In 2013 the Gospel Topics essay Race and the Priesthood disavowed the idea that dark skin marks God's disfavour.
Scripture Central, with the BYU scholars Ethan Sproat and Gerrit Steenblik, argue that "skin of blackness" may be a metaphor, or may mean garments and marks the Lamanites took on. On that reading the curse is separation from the Lord in 2 Nephi 5:20, and the mark is a separate thing.
FAIR adds that the book itself condemns Nephite racism — Jacob 3:5-9 calls the Lamanites more righteous. In some episodes the two peoples cannot be told apart, which a stark skin difference would prevent.
And the headings were 1981 study helps by Bruce R. McConkie, not scripture.
Strong for you, and this is one to make gently. The garment reading is recent.
It cuts against nearly two centuries of the church's own teaching, from Brigham Young to Spencer W. Kimball, who reported Lamanite converts growing lighter.
And the text's own chain resists it: cursed for iniquity, therefore a skin of blackness, so they would not be enticing.
"Read me 2 Nephi 5:21. What does it say the curse actually was?"

The mark may mean garments, and the church disavowed the racial reading in 2013.
Scripture Central and recent BYU work argue that 'skin of blackness' may be a figure of speech. Ethan Sproat and Gerrit Steenblik read it as garments and marks the Lamanites took on. On that reading the curse itself is being cut off from the Lord at 2 Nephi 5:20. The mark is a separate thing.
FAIR notes that the book itself condemns Nephite racism. Jacob 3:5-9 calls the Lamanites more righteous. In some scenes the two peoples cannot be told apart, which a stark skin difference would prevent.
The heading edits fixed 1981 study helps by Bruce R. McConkie, not the revealed text. And the 2013 Gospel Topics essay 'Race and the Priesthood' disavows the idea that dark skin marks God's disfavour.
Strong for you, but make it gently: the garment reading is recent, the old teaching plain.
The figure-of-speech and garment reading is a late development. It cuts against nearly two centuries of the church's own teaching. Brigham Young taught the plain reading. So did Spencer W. Kimball, who reported Lamanite converts growing lighter.
The text's own chain resists it too. Cursed because of iniquity, therefore a skin of blackness, so they would not be enticing. 3 Nephi 2:15 has the curse and the skin change together.
The 2013 disavowal grants the criticism against the plain sense. It does not refute it.