According to Mormonism, is the Black race cursed and if so what does that imply? I was wondering, since I've heard this a lot and I've done some research about it. I'd like to know what Mormons, at least contemporary Mormons, have to say about this. 


  Keeping in mind documents like these which I will quote to avoid
  needless opposition : 
  
  "Cain was cursed with dark skin; he became the father of the Negroes,
  and those spirits wh are not worthy to receive the priesthood are born
  through his lineage."
  (Mormon Doctrine, pp. 102)
  
  "Negroes in this life are denied the priesthood; under no
  circumstances can they hold this delegation of authority from the
  Almighty" (Mormon Doctrine, 10th printing, pages 527-528)
  
  "The gospel message of salvation is not carried affirmatively to
  them... Negros are not equal with other races where the receipt of
  certain spiritual blessings are concerned."(Bruce R. McConkie:
  Highlights From His Life &amp; Teachings. Horn, Denis B.(2000))


I will now UPDATE the preface because the previous quotes were seemingly not enough to avoid needless opposition. I am not doing this to simply mock, I am doing this to get a satisfactory answer meaning truth. 

Quotes from MORMON PROPHETS that are considered to be
infallible teachings of the LDS, or supposedly so, we shall see. 


  Because of [Cain's] wickedness he became the father of an inferior
  race...the mark which was placed on Cain and which his posterity
  inherited was the black skin. (Prophet Joseph Fielding Smith, the way
  of perfection p. 101, 107) 





  Some classes of the human family...are black, uncouth, uncomely,
  disagreable and low in their habits, wild and seemingly deprived of
  nearly all the blessings of the intelligence that is generally bestowed
  upon mankinnd. The first man who committed the odious crime of killing
  one of his bretheren will be cursed... the Lord put a mark upon Cain
  which is the flat nose and black skin.
  
  Shall I tell you the law of God in regards to the African race? If the
  white man who belongs to the chosen seed mixes his blood with the seed
  of Cain, the penalty under the law of God is death on the spot. **This
  shall always be so. 
  
  (2nd Prophet of the LDS Church, Brigham Young, "Journal of Discourses," Vol. 7, Pages 290 &amp; 291)





  The intermarriage of the Negro and white races, a concept which has
  heretofore been most repugnant to most normal minded people... There
  is a growing tendency...toward the breaking down of the race barriers
  in the matter of intermarriage between whites and blacks, but it does
  not have the sanction of the Church and is contrary to Church
  doctrine.  (8th Prophet of the LDS Church, George Albert Smith, letter to Virgil H. Sponberg (critic of the anti-black ban), May 5, 1947, quoted in Lester E. Bush, Mormonism's Negro Doctrine: An Historical Overview, p. 42)


Here are some links : 

http://www.mormonthink.com/QUOTES/blacks.htm

http://apostake.com/phicholmedici/racist-quotes-by-the-prophets/

http://www.mormonthink.com/QUOTES/blackintermarriage.htm 

https://www.fairmormon.org/answers/Mormonism_and_racial_issues/Racial_statements_by_Church_leaders

Please, if somehow the LDS's official Church teaching has changed, explain in your answer the difference between the old theology and the new theology, that is a prerequisit to get your answer validated, thank you in advance.
I will restate that the book Mormon Doctrine, written by Bruce R. McConkie, is not Mormon doctrine, nor ever has been.


  We [the First Presidency of the church] decided that Bruce R. McConkie’s book, 'Mormon Doctrine' recently published by Bookcraft Company, must not be re-published, as it is full of errors and misstatements, and it is most unfortunate that it has received such wide circulation. It is reported to us that Brother McConkie has made corrections to his book, and is now preparing another edition. We decided this morning that we do not want him to publish another edition1


Mormon doctrine on blacks and the priesthood can be found in Official Declaration 2, which in summary says:


  The Book of Mormon teaches that “all are alike unto God,” including “black and white, bond and free, male and female” (2 Nephi 26:33). Throughout the history of the Church, people of every race and ethnicity in many countries have been baptized and have lived as faithful members of the Church. During Joseph Smith’s lifetime, a few black male members of the Church were ordained to the priesthood. Early in its history, Church leaders stopped conferring the priesthood on black males of African descent. Church records offer no clear insights into the origins of this practice. Church leaders believed that a revelation from God was needed to alter this practice and prayerfully sought guidance. The revelation came to Church President Spencer W. Kimball and was affirmed to other Church leaders in the Salt Lake Temple on June 1, 1978. The revelation removed all restrictions with regard to race that once applied to the priesthood.


See also this article on Race and the Priesthood

On Mormon doctrine, mormonnewsroom says:


  
  Not every statement made by a Church leader, past or present, necessarily constitutes doctrine. A single statement made by a single leader on a single occasion often represents a personal, though well-considered, opinion, but is not meant to be officially binding for the whole Church. With divine inspiration, the First Presidency (the prophet and his two counselors) and the Quorum of the Twelve Apostles (the second-highest governing body of the Church) counsel together to establish doctrine that is consistently proclaimed in official Church publications. This doctrine resides in the four “standard works” of scripture (the Holy Bible, the Book of Mormon, the Doctrine and Covenants and the Pearl of Great Price), official declarations and proclamations, and the Articles of Faith. Isolated statements are often taken out of context, leaving their original meaning distorted.
  


--EDIT--

You mention Mormon prophets are infallible but James E Faust puts what mormonnewsroom (above) said clearer:


  We make no claim of infallibility or perfection in the prophets, seers, and revelators


Also, a bit of the history, of blacks and the priesthood, is found in the introductory paragraph of Official Declaration above (now in bold). I have heard this referred to as cultural doctrine. Because of racism during early church history, some of that bled into church practice and teachings but without any doctrinal foundation (remember church leaders are not infallible). Official Declaration 2 is the LDS church's official stance on blacks and the priesthood and was meant as a clarification for the church that had gone astray in this teaching.

1 https://books.google.com/books?id=xAm4qGyN2OYC&amp;pg=PA179&amp;lpg=PA179&amp;hl=en#v=onepage&amp;q&amp;f=false