Mormons are by definition the church of jesus christ of latter day saints and since i am one i can give the truth. blacks were allowed to hold the divine power as you stated or priest hood but we did not give it to many until most racial prejudice subsided. think of it if you did not like african americans and they had somthing in a church that you didnt you might get mad at that church. second we do belive in three levels of heaven and no hell, if you sined you go to a sort of pergetory for punishment then you get to move on to heaven. we do not have any affiliation with polygamy what so ever. joseph smith did not steal the gold plates, joseph smith did not see the religion out of a hat, what really happened is joseph smith read James: chapter verse 5 which say if you have a question ask god and hell give you the answer so he did, he found out that all religions had partial truth but not all of it and he was chosen to bring about gods full church to the earth. he was latter visited by an angel which showed him were the gold plates were and a translating device that worked by faith. since no one at that time had enough faith to read it he was the only one. the gold plates contained the book of mormon. a history of the people of jared(the people that fled from the tower of babel)and a history of the people of lehi. it is not any way shape or form added to the bible but it is its own book.
if you find a mormon they are usually nice people who are willing to help you.