I've seen two productions of the Wiz where there were very few, if any, African-Americans in the cast. Granted, these were at summer camps where diversity isn't exactly thriving.
As for that production I saw of Ragtime where almost no one was African-American, I will refrain from saying the name for safety/privacy reasons, but they did have a somewhat interesting concept. Because they did not have the diversity to cast the roles, they costumed each of the 3 main groups in their own like colors, and gave each group their own separate lighting to signify their differences.
I guess it's an interesting way to get past the whole race issue, but the show still should never be done if it cannot be properly cast.
-There's the muddle in the middle. There's the puddle where the poodle did the piddle."