My date just dropped a racial slur

I (45) have been friends with a woman (F45) for several years. We met through mutual friends, one of whom is a black woman. My date and I are both white Americans / Western European-heritage mutts. We are both on the progressive left politically. (Or so I thought).

We decided to try dating as we get along so well and are both attracted to each other. After several dates and her clearly showing lots of interest in me / fooling around/making out etc, she said "I just heard the funniest word, do you want to know what it is? 'Niglets' !" She then proceeded to laugh hysterically while I looked at her and just said "that's terrible and not funny at all". She was clearly embarrassed and stopped laughing, awkward silence ensued and she changed the subject immediately.

However I can't stop thinking about this. It runs contrary to all the values that I thought she and I shared.... I am 100% sure she would not have said that word so casually around our black friend. Which makes me think she harbors a secret, deep racism, and wants me to be in on it with her. I'm not on board with this. At the same time I appreciate her friendship and know she is not a terrible person although the way I'm starting to question everything. About dating her and were being friends with her. seems like a dealbreaker… any advice would be very appreciated, thanks!