I totally agree with what pillow and dragons said.
For minorities, EVERY day is filled with small incidents that remind you that some people are racist twats.
You just got caught in the crossfire. It wasn't your intention, you didn't even notice they were both black, but from her point of view, you were just another person pointing out that day that she is 'other'.
It's not fair that she shouted at you, but also it's not fair that people are racist twats to her.
Unless someone is a real bellend and has a massive go over nothing, I try to be forgiving.
As a white woman, I really didn't understand how it feels to be 'other' in terms of race til I moved to Asia. It is draining. It's draining to be asked every day if I can eat the local food, to have people be amazed that I can speak and write in the local language, to have people make assumptions every day, to be stared at every day, to have children yell 'look at that foreigner' every day. It's easy to say 'oh just get over it' or 'don't let it bother you' but some days, I have to admit, I HAVE been rude. The final straw for me once was someone praising me for being able to say 'hello' in the local language. I have been here for years, my husband is from this country, I have studied for years and they're just like 'OMG YOU CAN SAY HELLO'. They were trying to be nice but it just infuriated me and I was quite grumpy back at them.
It is so alienating. And I have only experienced it as an adult and not in my home country, so I can't imagine how shitty it must be for people to have experienced it every day for their entire life.