People can say and do things that are hurtful to other people, even when they don't realise it's hurtful.
Not surprisingly, children often do this because they are learning about stuff all the time. It's important for adults to help them make sense of things - including to explain to them how their words and actions might be hurtful, even when they don't know that or intend it to be so.
So yes, I can accept that a 10 year old wouldn't understand the connotations of "blacking up," but it's depressing that his parents didn't, either - didn't see it as an opportunity to talk through some of the issues surrounding racism.
Depressing, but predictable, that many are saying it's just harmless fun and people are getting too uptight about this.
Just because someone stomps on your toe by accident doesn't mean it hurts any less than if they did it on purpose. When you explain that the stomping actually hurts, you hope the other person will stop doing it. If they respond by insisting on their right to bounce around as they please, that you're making a big deal out of nothing, that it was okay because no harm was intended, etc., then you can't help but wonder if they actually care if they're hurting someone.
And yes, appropriating someone else's culture and identity for "shits and giggles" can be hurtful. I've done it before at Halloween when I was young - dressed as an Indian and as a gypsy. I had no understanding of the experience of either at the time or that what I was doing could be hurtful. Wouldn't do it now. Plenty of other costumes to pick from.