I don't think it's so much being oblivious as it is being continuously misinformed and held back from information. She was never told about what happened, and nobody did anything to rectify that; from where she stood Cloud was just a kid with an atitude that wouldn't hang out with them, but was nice to her.
It's a case of us knowing the truth and assuming she should as well. There's really no reason why she should have as well, and it's not like it negatively affected her feelings for him, since she fantasized about him as knight-in-shining-armor for years.