Earlier quoted context omitted.
"Not all were slaves, and writing letters" Your point? "Free" Blacks were exceptions (about 9% by 1850) not the rule. Preservation of their history too was an exception, not the rule. Controlling for literacy rates and economic deprivation suppresses the preservation percentage more.
The last Civil War soldier died in the 1950s. There were 4 million slaves freed in 1865. Plenty of time for people taking oral histories of primary sources. Besides, 9% of 4 million is 360,000. Quite a lot of people. On February 25, 1837, Cheyney University of Pennsylvania became the nation's first Historically Black College and University (HBCU). -- Google
It took quite a while for the US government to get around take oral histories from persons born into slavery, but it did so: https://www.loc.gov/collections/voices-remembering-slavery/a...