It's offensive because of the history and context.
IMO (I know, not that I deserve an opinion as a privileged white person) dressing up as a generic "black person" is different (and much worse, as it's reducing a whole lot of people to a stereotype) to dressing up as a specific person, as in the OP.
Like if it was "Dress Up As Your Hero Day!" and you wanted to go as Neil Degrasse Tyson because he rocks, it "should" be fine to paint your skin to look like him. But it's still not, because of the history of 'blackface' and the hundreds of years of systemic racism. Hopefully that changes one day, but it'll be many many generations away yet.