I'm sorry to hear about what happened and that woman was ignorant as fuck, but this kind of reaction really fuels the fire to some degree. People are dumb and ignorant and I don't blame your aunt reacting in the way she did, but I feel that hostility towards people like that usually does more harm than good. It only makes them more defensive and resistant to changing their views. If she'd said 'Well, their father is black. Is that something you find hard to believe?' or something along those lines, from a place of pitying her ignorance then I think there are more positive outcomes all round. You still let the woman know she's an ignorant piece of shit and hopefully make her question it, and for your aunt's own esteem and helping in not letting people like that woman get to her which is something the woman was likely after anyway by saying what she said.
Again, I don't blame your aunt for her reaction, and I probably sound somewhat insensitive and ignorant myself but I feel that any hostility and anger directed at people that are prejudice (not just racist, but sexist, homophobic etc) doesn't really help change attitudes but can hinder that change. Also I'm not saying that your aunt should take some responsibility for that woman's ignorance, it's just how I feel about telling people to fuck off and getting angry. Anger is natural, but I feel it's never really resolves anything.
If I'm wrong, feel free to tell me to fuck off.