There are many questions, misconceptions, and misunderstandings about Mormons, blacks and the priesthood. 
 
First off - due to the revelation on the priesthood in 1978 there are no limitations whatsoever for male, black members receiving the priesthood. 
 
Indeed the church is experiencing great success with members of all races - including blacks. 
 
We are all children of our Heavenly Father and we are all equal in His eyes. If there is any bigotry or racism among members of the LDS church - that is an individual problem with the person and needs to be repented of. There should be no tolerance for bigotry in Jesus Christ's restored church. 
 
If you want additional information - here are some additional links: 
- FairLDS information 
- BlackLDS.org 
 
To answer the original question - a "curse" in the gospel sense means separation from God. That was not the case. The Lamanites, etc were given a mark - they were not cursed. From the Blacklds.org website above - here is a quote from Sis. Renee Olson: 
 
"Brothers and sisters, black skin isn’t a curse. The curse of Cain was eternal separation from God. The curse was never being allowed back in the Father’s presence. The curse was knowing that he (Cain) had listened to the wrong voice and failed his mortal mission. That was the curse, not the skin. The dark skin was given as a protection. As my father has so beautifully taught me, black skin isn’t a curse, it’s a calling. I’d like to think that in the pre-existence, when told I had the opportunity to come to Earth, I chose this skin color just like I’m told I chose my parents (although I’m not so sure about that one). My father and I agree that we’ve become rather partial to our black skin and certainly hope we take it with us to the next life. After all, we make black look good!" 
 
Given that the mark was not a curse - there is no eternal ramifications and everyone will be judged for their own sins and not anyone elses sins.

Where not the whites also prevented from getting the priesthood at one time. ie: The apostasy. How come no one complaines about that?