The difference is power. Denigrating a politically weak group like black people is just bullying. Denigrating the dominant group is standing up to power.
Sure, which is why denigrating black people in media and corporate setting is completely acceptable, while denigrating whites is shunned and can easily get you fired. Oh wait, I meant the other way around.
Yup, you're sure showing all those poor whites in Appalachia and countless rural counties!
In other words, you're blind as to whether denigration is justified.
Not blind, but to make a really stark example, there's a difference between mocking someone when you have a gun to their head, and mocking someone who has a gun to your head. The former is just adding insult to injury, the latter is stupid but a little brave at the same time.
In the former example you have so much power that whatever cause you had to denigrate them no longer matters to you, but that is neither the common case nor principled.