Most researchers (by now) are aware of the role the Freedmen's Bureau played in registering marriages and issuing marriage certificates to the newly freed. If you have not found your ancestors in these records, look to the local county.
Some of the Southern states eventually passed laws regarding the registration of marriages at the county level for all of the former 'enslaved'.
Many of these marriages may be found in separate 'freedmen' marriage books. If you are lucky, you may come across records like the following example.
Be it Known that on the 30th of April 1870, before me ..... recorder and Ex Officio Not Pub in and for the parish of .... in the presence of the herein after named and undersigned witnesses, personally came and appeared Martin Cormier et (and) Leontine Montalbeau, former slaves who declared that in the year 1861 they contracted together a private marriage and that from said marriage were born the following named children to wit: Marie aged this day eight years, Emma aged six years and Celestine aged this day 4 years...
The appearers further declare that this declaration is made in order to accept the benefit of an act ... relative to marriages passed by the legislature of the State ... approved ... and in accepting the benefit of the act above referred to the appearers bind and obligate themselves to perform all the duties and to assume all the obligations imposed by existing laws in relation to civil marriages and to abide by the same.
and Recorded May 2nd 1870
Just before the above example:
... from said marriage was born the following named children to wit Adam born on January 25th 1865, Don Louis on the 25th July 1868 and Marguerite on 16th July 1869...[Ariole Jean and Estelle Lewis]
Just after the above:
... appeared Thomas Joachim and Angile Jackson ... from said marriage were born the following named children to wit ... Gustave born on the 16th day of October 1862, Augustine born on the 30th October 1864, Bertha born on the 3rd day of July 1866 [and] Osaire born on the 28th of August 1868...
The above was found in the Louisiana Conveyance records. It is not until one understands the importance of Louisiana notaries that this location appears 'natural'.
Conveyance records were mainly used to record the transfer (sale) of property. Notaries were used to record land transactions, donations (gifts), sales, emancipations and contracts (including marriage). Today's pre-marital contract would have been yesterday's entry in the Notarial Records. Children were 'recognized', estates were divided, previous marriages were 'named', family gifts were itemized in front of a notary - who was licensed by the state. Notarial records are legal documents.
Similarly, an understanding of each state's marriage laws and registrations will
Start by reading about the Freedmen's Bureau and Marriages.