Were you equally bothered by any comments made at any other times in the theatre or only the ones that you disagreed with or took personally?
Were they disrupting the film anymore than anybody else?

I suspect you took it as more of a personal attack than an off-color reaction of two ignorant school girls. If so, yeah you needed to keep that to yourself.

Had some white people jumped up and screamed something about black people while I was watching, my first thought would be "why are you here?" and my next thoughts would have been "How ignorant/rude. You're interrupting the film." Largely because 1. the hypothetical comment wouldn't be a personal attack, 2. because in order for someone to see the error of their ways they have to be open to the discourse regarding such. In the absence of that they are happy to hold on to it for whatever reason. Reasons of which are none of my business.

I would rather their ignorance not exist, I would rather that they had an epiphanous moment instead of falling back to what's comfortable, I would rather they see the merit in the moment. However in the absence of any hint of those things occuring I usually reserve my energy for more worthwhile endeavors. Silencing a person doesn't change their mind.