Anthropological information: The point of the "skin of blackness" is "that they shall be loathsome unto thy people." Note that in verse 23 the curse extends to those who might marry any of these people. Thus there is not only a geographic division between the brothers and their families, but a moral and religious chasm as well.
The question of marriage is an important one, because the Lord has now excluded a significant number of the potential marriage partners. The preferred mode of marriage would be to marry someone within the same tribe:
5 And Moses commanded the children of Israel according to the word of the LORD, saying, The tribe of the sons of Joseph hath said well. 6 This is the thing which the LORD doth command concerning the daughters of Zelophehad, saying, Let them marry to whom they think best; only to the family of the tribe of their father shall they marry. 7 So shall not the inheritance of the children of Israel remove from tribe to tribe: for every one of the children of Israel shall keep himself to the inheritance of the tribe of his fathers. 8 And every daughter, that possesseth an inheritance in any tribe of the children of Israel, shall be wife unto one of the family of the tribe of her father, that the children of Israel may enjoy every man the inheritance of his fathers. 9 Neither shall the inheritance remove from one tribe to another tribe; but every one of the tribes of the children of Israel shall keep himself to his own inheritance.
A marriage in the ancient world dictated the flow of rights and properties. In the case of Israel, it was not only physical property, but a religious inheritance that was passed on. Thus the injunction to marry within the tribe. The Nephites would have been inheritors of that tradition, and would therefore prefer to marry within the tribe. However, half of the potential mates had been cut off and prohibited.
The preference for marriage inside the tribe was coupled with other prohibitions against marriage outside the tribe:
This analysis of the marriage laws is important to our understanding of the developing Nephite society. Not only is their tribe split in half, but by this time each person was related to the other by blood or marriage. As noted in Smith's passage, this would create tremendous difficulties in maintaining the Law.
Three points become important. The first is that marriage outside the tribe, indeed outside of Israel was permitted but discouraged (with very specific prohibitions against the Canaanites, and legal prohibitions for marrying *men* from the Ammonites or Moabites). There is even the ability for men from outside of Israel to be incorporated legally. It is therefore quite likely that in order to continue the tribe, the Nephites and Lamanites were required to marry outside of their small group. This inevitable intermarriage with other natives of the land is the best way to account for the impressive increase in population noted in the Book of Mormon. While not the preferred method when the larger population of Israel was available, it was nevertheless allowed, and preferable to the more direct violation of the prohibition of marrying within ones immediate and near family (which certainly would have described the original Nephite and Lamanite bands at this point in time).
The second point is that the prohibition of intermarriage with the Lamanites follows the long Israelite tradition of prohibiting marriage with a particular people. In the case of the Lamanites, there was likely just as much potential of disruption of the religious tradition of the Nephites as there was with the Canaanites.
The third point of interest is that the prohibition against marriage with the Lamanites comes very early, and at this point is certainly directed at the specific tribal affiliation of Lamanites/Lemuelites, etc. When the term "Lamanites" appears to become a more generic term, akin to "gentile" it is likely that the specific prohibition was lessened, although it might have remained in place with the lineal descendants of these original Lamanites.