Why do you assume she was reacting to your being black? Did she know it was you, or did it take her a second to recognize you? Does she have any history of being the victim of a crime, especially in the establishment of which you speak?
In a vacuum, if the situation was exactly as you described, perhaps your response is warranted. However, since we can't accurately establish her state of mind at the time we are grasping at straws, and ironically it is you who has jumped to conclusions and presumed her reaction stems from prejudice due to ethnicity. Sit back and think about it for a bit. Maybe you're right about what happened to the letter, but for the time being, let's give her the benefit of the doubt that you would want if she were pre-judging you.