Sebesta is no Lerone Bennett, I admit. But without knowing Blackerby or his source I won't say that he is any worse than Sebesta or any better.
I can't say I know much about the first either, however the law of probability dictates that he is not likely to be worse than your source of choice since Sebesta falls somewhere on the left wing wacko tail of the population distribution. I'd estimate him at somewhere to the left of Chomsky and Mary Frances Berry, but just to the right of the absolute far left nut fringe, aka Cynthia McKinney.