BEN - As I was talking with some family about the Church, the topic of blacks and the priesthood came up. It was said that, until President Kimball kneeled in prayer concerning people of all races being able to hold the priesthood, it had never before been revealed that those of African descent could not hold the priesthood, and that this was the first time such a revelation had been made, that the restriction on the priesthood was simply assumed given scriptural reference and culture at the time. I then realized that I hadn't, in fact, read any canonized, official church statements barring blacks from the priesthood. Is this true, or is there something I've missed?

JOEL - As you probably know, there are verses in the Book of Abraham, which may have been the basis for the priesthood ban. In the first chapter of the Book of Abraham, Abraham writes that Pharoah, the king of Egypt, was "a righteous man", but could not hold the Priesthood because he was a descendant of Ham, and the Hamitic lineage or bloodline was "blessed with wisdom" but "cursed as pertaining to the Priesthood" (See Abraham 1:26).
In another revelation in the Book of Moses, it says that the Cainites, the descendants of Cain, the son of Adam, were "black" (Moses 7:22).
From the early days of the church some members have interpreted these verses as saying that blacks, being of the Cainite/Hamitic bloodline, would not be allowed to hold the Priesthood until the Abelites first had the opportunity.
On January 6th, 1848, based on the Abraham scriptures, Brigham Young declared:

"The Lord said I will not kill Cain, but I will put a mark upon him and it is seen in the face of every Negro on earth. And it is the degree of God that that mark shall remain upon the seed of Cain and the Curse until all the seed of Abel should be redeemed and Cain will not hold the priesthood until all the seed of Abel are redeemed. Any man having one drop of the seed of Cain in him cannot hold the Priesthood and if no other Prophet spake it before I will say it now in the name of Jesus Christ." (Journal of Wilford Woodruff 4:97)

Even though the ban started in earnest with Brigham Young, Joseph Smith did once refer to Negroes as "the sons of Cain" (History of the Church 4:501) and statements of associates of Smith suggest the the doctrine started with him posibly based on the Abraham scriptures.
Later in Church history on August 17th, 1949, the First Presidency issued its official "Statement by The First Presidency of The Church on the Negro Question" which said in part:

“The attitude of the Church with reference to Negroes remains as it has always stood. It is not a matter of policy but of direct commandment from the Lord, on which is founded the doctrine of the Church from the days of its organization, to the effect that Negroes may become Members of the Church but that they are not entitled to the Priesthood at the present time."

Another official statement from the Church presidency in December 1969 titled "The First Presidency on the Rights of the Negro" says the following:

"From the beginning of this dispensation, Joseph Smith and all succeeding presidents of the Church have taught that Negroes, while spirit children of a common Father, and the progeny of our earthly parents Adam and Eve, were not yet to receive the priesthood, for reasons which we believe are known to God, but which He has not made fully known to man.
Our living prophet, President David O. McKay, has said, "The seeming discrimination by the Church toward the Negro is not something which originated with man; but goes back into the beginning with God....
"Revelation assures us that this plan antedates man's mortal existence, extending back to man's pre-existent state." President McKay has also said, "Sometime in God's eternal plan, the Negro will be given the right to hold the priesthood."
Until God reveals His will in this matter, to him whom we sustain as a prophet, we are bound by that same will. Priesthood, when it is conferred on any man comes as a blessing from God, not of men."

Even though the origin of this doctrine is a little hazy and there were no "official" written revelations or scriptures pronouncing the ban, sources indicate that this doctrine was not just "assumed" from the scriptures, nor was it just opinion; it was declared in addresses by early church leaders, based on their God inspired interpretation of the Abraham scriptures mentioned, and it was supported as official Church doctrine by later prophets of the church.
For these reasons, this doctrine would not be considered just an unsupported man-made policy or "mistake" that needs to be admitted to.

"To every thing there is a season, and a time to every purpose under the heaven:" (Eccl. 3: 1) For a purpose known to God there was a season of time that the Blacks were to not have the priesthood; just as there was a season of time that the gospel was not to be preached to the Gentile nations or Samaritans(Matthew 10:5-6).

Updated statement from the Church on Race and the Priesthood (2013)