Verse 23 is fascinating in the disparate time frame of the "dark, and loathsome" in this verse and the "skin of blackness" mentioned in 2 Nephi 5:21. The curse of the "skin of blackness" comes very soon after Lehi's death in the New World, and yet in the prophecy, the remains of the seed of his brethren are not "dark, and loathsome" until many generations after the demise of the Nephites, and the total withering and unbelief of the remnant.
It would appear that verse 23's "dark, and loathsome" does not refer to skin color, where the "skin of blackness" is clearly related to pigmentation. The extreme pejorative descriptions of 1 Nephi 12:23 are not associated with skin color, but with a spiritual state.