My feeling is: black people have made it very clear they'd appreciate if you don't wear black face.
So even if you don't get it or don't totally agree: why disrespect black people in this way? Like, what, it was really important that your costume be 100% accurate?
Where are all these black people saying how horribly offensive it is? Because the overwhelming majority of the time it's upper middle class white people trying to look after black people by calling other white's racist whenever this comes up. Out in the real world this seems like an issue where no one really gives a shit: blacks, whites, asians, etc don't give a damn what you dress as where I live. Not one time in the real world has anyone I've ever spoken to, of many various backgrounds no less, told me they were offended by a Halloween costume.
It's a single day out of the whole year where you're expected to dress up as someone you're not, and the fact that over a hundred years ago it was racist to paint your face black doesn't make it so today. It's not the 1800's and early 1900's anymore, it's fuckin 2015 for christ sake. How much longer till it's deemed okay? This kid isn't trying to appropriate black culture any more than a white person dressing as Dracula is trying to authentically appropriate Romanian culture.