Here are the indisputable facts.
388,000 slaves were brought to the United States of America before 1866.
US Population in 1860 was 31,443,321.
(So blacks started out as 1.2% of the population)
In 2010, Blacks were 12.6% of the United States of America's population (total 308.7 million)
It's not that the slave ships were running overtime bringing millions of blacks into the country. It's just that they have been more fruitful and multiplied more than their Protestant white neighbors who had other priorities. (For what it's worth, I criticize those other priorities for the empty stupidity that they were.)