It’s just as reasonable to assume that he’s reacting based on her being a “bitch” or that he’s condescending to her because she’s a woman as it is that he’s reacting based on racism.
There may be no reason for her character to be secretive, but it’s a requirement of the story that the blanks be concealed. You can argue all day long over how effective he was in telling that bit of the story, but it’s irrational to cling to a claim for which there is not only evidence in the film that refutes it but that the writer/director refutes.
But her choice isn’t half-assed. Her intial desire was that her father not get a gun. The store owner refused the refund so she was stuck with the gun. From her perspective, any (live) ammunition was equally bad, because the gun was bad. So yes, at that moment she doesn’t care. A moment later, when she sees the box of blanks and decides on them, obviously she does care.
But it doesn’t convey the opposite. It conveys that she knew, through her fixed glance on the box.
Stand away. Keep on arguing over whether or not she knew when the creator has made it clear through his commentary that she did and included information that indicated she knew (her gaze fixing on the box of blanks) but it makes no sense to do so. The argument is over how well he conveyed it, not over her knowledge, because, sorry, she knew.