You may do well to read the discussion under "Why did mormons abandon polygamy?" 
 
As well as "Are you sure Joseph Smith and Brigham Young are true prophets?" 
 
The Lord calls Imperfect men to lead his church and to act in his stead. They are still entitiled to their own opinions and views on certain things. They however, are not allowed to speak these Opinions and views, and declare them as official stances of the Lord's church. If they did, then the Lord would remove them from their place. I believe that the discussion on the Polygamy question will be sufficently answered through that discussion found under the question on this site where that is addressed. 
 
As for the Blood Atonement portion, that has never been declared as an official doctrine of the church. Of course people, have their own opinion, (I'm sure you have your own views and opinions when it comes to different aspects of your religion, (or your atheistic beliefs) as well.)which obviously has differed through the years, but has never been pronounced as Doctrine. When the Prophet is speaking for the Lord, the Prophet is 100% correct, 100% of the time (Thus saith the Lord).  
 
The times that the Prophet is speaking for the Lord, is declared as the official doctrine of the Church. Official Doctrine of the Church can be found in the Scriptures (Old Testament, New Testament, Doctrine & Covenants, Pearl of Great Price, Book of Mormon). As well as the things spoken from the Pulpit during General Conferences of the Church. 
 
Out side of these "Official Source" There have been many things that have been attributed to be a part of the Church's Doctrine, by those that are not allowed to designate such as Doctrine. To know if something is actually Church Doctrine, one would be wise to consult the Scriptures, as well as Conference Addresses.

Simply put, Practices can change, but Doctrine is eternal. 
 
Practices can change as the Lord reveals things through His servants, the prophets. This shows how important it is to have ongoing revelation; someone who speaks on behalf of God. Many points of the Law of Moses were practices that were fulfilled with the death of Christ. 
 
However, every practice originates from a doctrinal parallel. The doctrine of the Gospel of Jesus Christ is unchanging, even though true followers of Christ in each dispensation practiced according to the revelation they received. 
 
For example, in Old Testament times, sacrifices were offered in the tabernacle to remind the Children of Israel of the coming of the Messiah, and of the Atonement. In original Church, the sacrament was instituted as a reminder of that great atoning sacrifice. The doctrine of the Atonement remained constant, but as the time period changed, the practices changed. 
 
I'm glad that God continues to reveal the order of certain practices to living prophets, and I'm glad we have their words and the scriptures to establish the doctrines of truth.