The word "ballad" has different meanings. In a folk music context, a ballad is a song that tells a story. In a pop music context, a ballad is a slow song.

I suppose that, depending on the orientation of the person using the word term, a blues ballad might mean a blues song that tells a story, or a slow blues song. When Alan Lomax labels a song as a "blues ballad," I would think he's referring to a story song. When Robert Christgau uses the same term in the Village Voice, I assume he's talking about a slow song.