Dressing as an entertainer you admire for Halloween could be well-intentioned, but at least in the US, it's very hard to divorce from a really odious history.
I think what a lot of people are forgetting here is that in the American context, the division between "blackface" and "costumes that incorporate blackface" is completely inseparable. Without writing an entire essay on the history of minstrelsy in America, I do want to acknowledge that at points of the 19th Century, blackface minstrelsy was a dominant, if not THE dominant popular entertainment medium (https://en.wikipedia.org/wiki/Minstrel_show
. A century and a half later, blackface is probably one of THE GREATEST taboos of American culture, right alongside the KKK and fascist iconography. Blackface is outside the realm of acceptability in the US, and even films that featured blackface with a degree of irony, like 2008's Tropic Thunder had to tread very carefully. I understand that blackface in other contexts and locations may be less loaded, such as Zwarte Piet in the Netherlands, but in the United States it's a relatively unquestioned taboo.
I think one of the main issues people are taking with Kelly isn't necessarily what she directly said, but the fact that she is putting this up for debate, despite 60-odd years of blackface being regarded as simply beyond the pale in American society. It's a pretty well accepted "no-no", so having a mainstream, centrist network give airtime to this was enough to cause this furor, both within and outside the scope of the network. It's not a matter of "not wanting to debate both sides", blackface has been well outside the Overton Window for decades, and I think the outrage is justified.