FYI, the Church of Jesus Christ of Latter-day Saints never said that blacks could not attain salvation. That's a really unfortunate mis-representation. The policy was specifically a ban on holding the priesthood, which is very, very different.
Also, the Book of Mormon was published in 1830. Smith didn't say it was a dictation (you're probably thinking of the Koran, which Mohammed said was dictated to him by the angel Gabriel); he said it was a translation of ancient golden plates he had been directed to by the angel Moroni.