You SHOULD be surprised, except we've gotten to the point that any little thing can be offensive. And she didn't just paint her face black, she did stuff with her hands too, and also her whole custom and hair style.
I've already stipulated that it's not exactly like "blackface." That's no reason to pretend this is like a banana peel in a tree. Sometimes, in the real world, innocent things offend people. Sometimes you quit doing that thing, just to be considerate. How terrible.
And that's also ignoring that actual blackface isn't racist to begin with.
I don't know whether or not that's true, but just for the sake of argument, I'll go along with it. If true, it's like a lot of allegedly racist terms, such as "Negro" or "Jap." There's nothing racist about those terms, but you know if you use them, folks will be offended. Most adults (I think) still know what "bastard" and "bitch" literally mean, but I know people will be offended if I use their literal meanings in conversation. So I generally do not. There's no percentage in it.