The U.S. doesn't claim to be the most virtuous country in the world, that would be Canada. 
How would anyone know what atrocities have been committed against blacks in Canada?  The news of today is heavily censored in Canada to keep the bad stuff out. Do you seriously think you're going to hear about past unpleasant events? Do you think the detainee scandal will be in Canadian history text books?
Blacks in the U.S. at 13% of the population are empowered while in Canada they are beholding to the majority and evidently that's not working out too well.
If you read the article in the G&M on the topic, or the actual UN report, you will see that mistreatment of blacks in the U.S. isn't considered a plus for Canada. In fact, the report is about Canada and it takes all the self-serving flattery you heap on yourselves and blows it apart.