The film by Marlon Riggs is called Ethnic Notions.  It deals with Minstrels
in a very broad sense, but there is a segment in which choreographer Leni
Sloan does a kind of homage to Bert Williams.  Depending on what your
interest in minstrelsy is, you might find this useful.
 
Jennifer Miller
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Online resources for film/TV studies may be found at ScreenSite
http://www.tcf.ua.edu/screensite