You mean there were some black people in colonial days who who voluntarily got on the boat in Africa, arrived in America as free men, and who remained free, and whose descendants remained free straight through to the end of the Civil War? Or do you mean there were some early slaves or children of the first groups of slaves who gained freedom very early on during Colonial days?
The earliest blacks came as indentured servants who became free when they'd worked out their contracts, as did white indentured servants. Some were from Dutch slaveships but became indentured when they came ashore, since slavery was not supported by law. That unfortunately came later. They were not afforded the same rights but some did prosper and were able to afford their own indentured servants. The prosperous blacks were more likely to remain free, and I've read about one who won a lawsuit about a black slave of his who had been stolen by a white man.
When the supply of indentured servants became low, slavery became codified into law and children of slaves were automatically slaves. In Virginia the status of the mother determined the child's status for a time and there are instances of slaves and indentures producing indentured children who worked their 31 years and then were free.
Some, like the founder of Chicago, Du Sable, were of French and African (slave) blood, but they came to N. America as freemen. He was one of the black French fur-tappers I'd heard about some time ago.
Not all blacks at the time of the Civil War were slaves.
Interestingly, the word "slave" comes from Slavic people being a large group subjected to ownership by other Europeans. It's very possible that quite a lot of us have some form of slavery in our ancestorship, no matter what color we are.