One HUGE difference is that the term "sheep" describes a mindset that people have either chosen or have fallen into by default while the other is a term that denigrates someone for the way they were born.Reminds me too much of a KKK talking about "n******". In fact, I'm not sure there is any difference.
People can stop being "sheep" if they so desire.
Given the total impossibility of creating a reasonable argument to support the assertion that person can wake up one morning and decide to stop being black, it should be plain that your analogy is fatally flawed.