As someone who's never been to the USA I might be making a completely wrong assumption, but from all the media, it seems that the majority of the African-American population were traditionally Christian. Knowing that most of their ancestors arrived as slaves from Africa, they could not have been Christians upon arrival(right?). Was there a revival or some other major event that brought them into Christianity?
Pretty much the classic study of the African-American Church in America is E. Franklin Frazier's 1964 The History of the Negro Church in America. Indeed, its popularity can be evidenced by another book, The History of the Black Church since Frazier.
In a nutshell, the history of the black church in the United States is what you'd expect:
The interesting part of the story is the motivation and whether the movement was "forced" upon Blacks or whether it was a blessing coopted by them.
Ever since 1619, when African slaves were imported to Jamestown, the slaves were the nucleus of Christian training and education. The cynical will say it was a means of controlling the population, and the sociologist may argue that the church was written onto the cultural tabula rasa that arose in the absence of any other common tradition or language. (Per Frazier, the American slave population was drawn from the vastly different regions of the slave coast, and thus there was no single common identity from the Old World.) And, to be sure, there were "purer" motives of "saving the heathens" - both paternalistic and out of genuine concern.
A modern missiologist, however, would most likely argue that the history of the black church is the history of Joseph - what [we] intended for evil, God used to work for good. Indeed, Frazier argues the church rose as much for economic motives (replacing the tribal cultural an massing the strength of the African American community) as religious ones.
But to merely ascribe the rise of the church to "population control" motives misses out on the fact that the black population truly made the church their own is to do African Americans a major disservice. The story of Richard Allen, Absalom Jones and the rise of the AME church, for example, shows that it was not something "forced" upon them. In the 1780s's, Jones and Allen led a black congregation that met in St. George's Church in Philadelphia in the early morning. Jealous of the success of that church, the white vestry that owned the building made life progressively more difficult, culminating in an episode in which Allen, in the midst of leading prayer, was savagely beaten at the altar. His congregation moved out and later helped nurse Philadelphia through an outbreak of yellow fever - in spite of the opposition of the establishment.
As to the amazing rise of the church, it should be remembered that much of the slave trade - muchly stopped by the British who made it illegal to transport slaves across the Atlantic in 1808 - coincided with the First and Second Great Awakenings of the 1700s. The fact that most African Americans today are Methodist and Baptist is indicative of that fact. (Remember, they were imported into a colony - my own Virginia - that was strictly Episcopalian, and the "E" in AME is Episcopalian, but that "M" is Methodist). The overall rise in religiousity in American culture simply spilled over across racial lines.
I have no argument that slavery was anything less than unmitigated evil. That it was even justified in theological terms (Cursed be Caanan / Gen 10) is reprehensible. And indeed, if you look at how segregation in churches was the response to the loss of the Civil War, I think it difficult to justify. But, in the same way that God used the sins of Joseph's brothers to glorify Himself, so to the rise the church that was introduced at the lash of the whip came to change the consciousness of the slave owner. From the Christian Abolitionist movement of the early 1800s to the Civil Rights era of the 1950s (note, there is a reason Rev. Martin Luther King, Jr called it the Southern Christian Leadership Conference, the issue of the "black man" was radically redefined when understood in the light of Jesus. To call the church "forced" upon blacks may have been true for a very short period, but they truly made it their own.
Here is some excerpts from some research I have done for a book I am writing.
It appears that Slavery began at about the time people began to spread out in ths American Clonies.
Slavery spread quickly in the American colonies. At first the legal status of Africans in America was poorly defined, and some: like European indentured servants, managed to become free after several years of service.
English law in the colonies until 1776, and the Revolutionary war, provided that any slave, who converted to Christianity, would be indentured and would; after their owner's expenses were repaid by servitude, become free citizens, and most were after fulfilling those requirements did convert to Christianity.
Christianity spread quickly among slaves, most of whom were from the African Nations, since those were the nations in which tribal life continued and there was a lack of centralized Government.
Many black Churches were formed by those freed during the colonial period out of possible fear that former owners would try to reclaim them as not really converting.
After 1600 many states passed laws making Slavery permanent, however the Christian movement among the Blacks had by this time gained much momentum, and slaves quite often would gather outside the open windows of their master's churches, and listen to the Sermons.
After the Civil war Christianity abounded among the freed slaves, and in the south where almost no freed slaves were literate Many used what is now called the Cotton patch Bible.
If you are interested there are a multitude of sites on the internet covering this subject. A simple Google search for Black Christianity history will lead to many different sites.