Ah yes I was praying
for you to bring up the heavily misunderstood black issue. I almost don't blame you because most members don't study their doctrine enough to answer this question. I fortunately needed to know the answer to this question to be able to rest in peace about it. So I wouldn't stop researching the subject until I was satisfied. Pay close attention cuz I think you might be tempted to gloss over some details. I don't want to refer you to this again because it should be fully explained here.
I served my mission in South Africa, and so naturally would get this question often. In the book the Pearl of Great price, part of which is a translation from some ancient egyptian scrolls which came into the hands of Joseph Smith, there is a book called the book of Moses and the book of Abraham. In the book of Abraham, there was described the punishment of Cain for his crimes. he was cut off from the Lord and was cast into a land separate from the other children of Adam. It was then because of Cains transgression, the LAND in which he lived was cursed with much heat, which we now know to be the land of Africa. We do believe that original africans were descended from Cain but don't get twisted into a bundle just yet. Cain and his children then had a "mark" placed upon them to distinguish them from other children of Adam. this mark was described by Brigham Young as the dark skin and flat nose. Still not time to get riled up yet.
The curse was not to be confused with the mark. It is nowhere described that the descendants of Cain were cursed, only marked. In fact this marking was a huge blessing for a couple of reasons. The LAND being cursed with much heat was and still is a very inhospitable place because of the sun exposure. How on earth would they be expected to survive such conditions without God preparing them for their enviroment. As we all know, dark skin is a built in sunburn defense. and yes I know black people can sunburn too, but certainly not to a fried crisp like if I were to stand out there for 15 minutes. The flat nose was also a blessing. we all know that hunting in africa is difficult and requires lots of running and stamina if you are going to track and hunt prey in the african wilderness. hence as we all can see, most of the best and naturally gifted long distance runners and sprinters are africans, the nose shape playing a huge role in the amount of oxygen they are able to breath in and out of their nostrils to keep up such a pace. Can white people be trained to run long distance. Of course, but they have to work a little harder to develop the same level of stamina because of the shape of the nose. Starting to make sense?
It gets better.The reason a lot of mormons and non-mormons alike confused the skin color of Cain's descendants with a curse is because of an UNRELATED and temporary physical curse placed on the Lamanites to distinguish them from the Nephites and cause that they should not be enticing unto the nephites during much of the Book of Mormon years. The curse we read in 3rd Nephi was eventually taken away and had nothing to do with having a flat nose. It is easy to see how this can be misunderstood through lack of study.
No where is it written in our official doctrine that blacks were cursed with dark skin as a result of poor performance in the pre existence. That unfortunately has been misunderstood by too many members and apparently you too.
Next question please.
Not everyone is as ignorant of your faith's history as you are. The first time I read the BOM, it still included the words "white and delightsome," though I understand those are no longer in the book. 
I think it's gone through a few revisions since my day to get to the sanitized version you currently cling to.
