I don't think they can.
The deeply racist message of the Book of Mormon isn't at bed rock with the claim dark skin is the sign of a curse from God. If so, one could still arguably claim this was an artifact of pre-Columbian beliefs held by literal Nephites. This is just a narrative expression of the racist beliefs in the Book of Mormon borrowing from the Bible.
The actual bed rock racist message of the Book of Mormon is that there was no way the Native Americans and their ancestors could be responsible for the advanced civilizations evident to the early European migrants. Native Americans were savages, dirty, wild and uncivilized. This is irretrievably embedded in the period of European occupation.
This was a belief that Joseph Smith didn't invent but was wide spread in the 19th century. The Book of Mormon mainstream and keeps fresh a belief from that era that has otherwise died out both for being factually wrong and for being abhorrent to reasonable people.