My progressive friend looked at me and wearily said "Right now, any white guy who dresses up like a black guy is doing 'blackface.' " He doesn't get it either, but he says okay; he's a party man.
Look, it isn't like I stay awake at night thinking up ways to offend others. But this is beyond reason. Who are black people to have a rule that none may don a costume pretending to be a black performer? I am NOT talking about blackface. Explain how Herring is an example of blackface and why Blow is so outraged.
Here's a concise history of "blackface", the degrading legacy it entails, and how it became part of American culture:
Why Won’t Blackface Go Away? It’s Part of America’s Troubled Cultural Legacy
Without that background, yes, the current reaction to Northam would be incomprehensible. Including it, and 150 years of denigration and humiliation, into your deliberations should help explain why blacks are wary of whites dressing up as black. That's the most salient part.
There are at least two additional aspects worth consideration: First, there's the the Culture of Outrage, playing out mostly on so-called "social media", with folks jumping at whatever click bait dangled before their noses.
Second, and maybe more importantly, there's the ratfucking by the right, trying to make up for their losses in Virginia, and desperate to distract from their having the most overtly racist goon in many decades elected to the most powerful office of the land. And that's why they need to amplify the aforementioned "social media" hyperventilation to the max.