Slaves were considered property and so their "marriages" weren't legally recognized.

Some slave owners would allow their slaves to perform marriage ceramonies and form families. But those marriages were not recorded as such.

After emancipation, the federal government established the Freedmen's Bureau to help former slaves get established in the society as free men. One of the services provided by the Bureau was to record marriages that had taken place during slavery. Those records are available to researchers, and can be a valuable tool.