Here's a more recent clip of Jesse Jackson, addressing a conference. I haven't listened to more than a few minutes of this 42 minute clip, but from what I've heard it seems perfectly clear and articulate and easy to understand to me.

There are different styles of oratory, and the preacher style (black or white, American or whatever) is just one among them. Real orators know how to use the appropriate style for the audiences they are addressing, and for the mood of those audiences.