thank you @umbrarchist for inspiring this. I have known of gutenberg for years and used it but never thought to go through its library for Black literature. 

Books in African American Writers (sorted by popularity)
https://gutenberg.org/ebooks/bookshelf/6
Books in African American Writers (sorted alphabetically)
https://gutenberg.org/ebooks/bookshelf/6?sort_order=title
Books in African American Writers (sorted alphabetically by author)
https://gutenberg.org/ebooks/bookshelf/6?sort_order=author
Books in African American Writers (sorted by release date)
https://gutenberg.org/ebooks/bookshelf/6?sort_order=release_date

I haven't gone through the books , I have merely listed ther front pages , but I will go through them and make various points later. 
 

What is the oldest Black written book you have read?

I have read part of the kemet book of the dead, the oldest black book from the global black populace i can think of. 

The oldest Black DOS book I read is every tongue has to confess by zora neale hurston as it contains the best collection of black fiction/fantasy that is also enslaved narratives. Most enslaved narratives are biographical, they don't deal with fiction or fantasy. 

It will be interesting to see if any of the Black DOS enslaved narratives are part of gutenberg. I will have to do this later:) 
https://en.wikipedia.org/wiki/Slave_narrative