Indeed there is. There is one person in particular, Omar ibn Said, who is a somewhat well known example of an African Muslim in that period as a clear example of an African slave who was a Muslim. He was able to write an publish an autobiography some of which you can read online. You can also see a page from the text here, which shows a chapter from the Qur'an written by ibn Said.

The following is from his biography:

My name is Omar ibn Seid. My birthplace was Fut Tûr, between the two rivers. I sought knowledge under the instruction of a Sheikh called Mohammed Seid, my own brother, and Sheikh Soleiman Kembeh, and Sheikh Gabriel Abdal. I continued my studies twenty-five years. Then there came to our place a large army, who killed many men, and took me, and brought me to the great sea, and sold me into the hands of the Christians, who bound me and sent me on board a great ship and we sailed upon the great sea a month and a half, when we came to a place called Charleston in the Christian language.

Quoting from Wikipedia for brevity:

Most of Said's other work consisted of Islamic manuscripts in Arabic, including a handwritten copy of some short chapters (surat) from the Qur'an that are now part of the North Carolina Collection in the Wilson Library at University of North Carolina at Chapel Hill.

I'm not sure what the actual numbers were, and hopefully someone can provide some proper account, but at least one suggestion is that it was around 15%. What his case tells us though is that it was happening, and that in the early 1800s there most definitely were Muslims among those enslaved.

Hope that answers your question. There's actually a decent segment on a pоdсast called Backstory that's worth a listen if you're interested.