You are correct. Such a film was released in 1918, called "The Birth of a
Race". It was produced with funds raised by public solicitations, evidently
spear-headed by a man named Emmet J. Scott, a former secretary to Booker T.
The film had a rocky history and the result was a film that pleased nobody at
the time (more than two years after the release of Griffith's film.)
I believe only a portion of the film survives today. For a detailed history
of its production see "From Sambo to Superspade" by Daniel J. Leab - 1976,
Gene Stavis, School of Visual Arts - NYC