This is something easy to overlook due to the Western bias in how we Westerners look at History, including Christian History.
And I feel I can document this Independent of The New Testament's own claim of a Eunuch of a Candice of "Ethiopia" becoming a Christian in Acts 8 (Amantitere was probably the Kandake at that time) within the first few years of the Church's existence.
And I can also say It's still true even if you don't count Roman Occupied North Africa (including Egypt), where it's difficult to be certain of anyone's exact ethnicity. I'm confident the Christian Communities of those provinces included representatives of all their Ethnic groups (I did a post on Perpetua and Felicity already), but I'm gonna stick to indisputably Black Africa for this post.
The first Christian King in Africa was King Ezana of Axum. Since he lived in the 320a-360 AD, he's post Constantine but still before Christianity became fully Rome's state Religion under Theodosius I. It's safe to say Christianity must have existed among the population in some capacity already before then.
King Ezana was also the first Christian King to use a Cross on his coins. Constantine's preferred Christian symbol was the Chi-Rho. Ezana was also specifically Nicene, the Arian Emperor Constantinus II wanted to question him and his brother on matters of doctrine, but they ignored that request.
That Branch of Christianity still exists to this day.
Lesser known then that however is the history of the Church in Nubia. Here is one interesting YouTube video on the subject. This guy also wrote a Book Roots of Nubian Christianity Uncovered: The Triumph of the Last Pharaoh.
The king being refereed to in a very non literal sense as the Last Pharaoh here is King Silko who reigned in the 5th century. There is some information about him here Temple of Kalabsha.
The last phase of the establishment of Nubian Christianity was three Kingdoms becoming officially Christian in the 6th Century AD. Nobatia in 543 AD, and then Makuria, and then Alodia in 580 AD. Two of those three Kingdoms lasted nearly a Thousand Years.
I could just as easily have called this "Christianity was firmly established in Africa before Islam existed". But it's not my intent to attack African American Muslims, or even the Nation of Islam which I do like to say are to Islam what Mormonism is to Christianity. The specific tribes who were enslaved by the Trans-Atlantic slave trade were neither Muslim or Christian before being enslaved, they were predominantly taken from a totally different part of Africa.
But I do sometimes feel like if African Americans want to reject the Faith their Slave masters forced on them, but remain Monothestic/Abrahamic, they should maybe consider converting to Miaphysite or Nestorian Christianity. Most of these early African Christians I just talked about were Miaphysite, and the Nestorians of East Asia are known to have rejected Slavery. The Trans-Atlantic Slave Trade however was created and run by Catholics and Anglicans, who are all Chalcedonian.