If Taylor Swift wants to end the bad blood between she and Katy Perry, she's going to have to do this, first.According to E! , when Perry was asked by a fan on Twitter if she would ever collaborate with Swift, her answer was simple: "If she says sorry, sure!"That apology from Swift would be for her song "Bad Blood," which reportedly is about Perry "stealing" Swift's backup dancers.While Swift has never officially confirmed the 1989 track is about Perry — she told Rolling Stone in 2014 it was a about a "pop star" who stole her dancer — the California girl definitely thought the song was about her. Perry tweeted shortly after the Rolling Stone profile, "Beware the Regina George in sheep's clothing..."But Perry may be waiting a while for an apology from Swift, who is known to write songs about exes and others she's not too fond of with reckless abandon. Though she'll never confirm or deny who these songs are about, telling GQ last year, "The fact that I’ve never confirmed who those songs are about makes me feel like there is still one card I’m holding."She's allegedly written songs about ex-boyfriends, such as Harry Styles, Jake Gyllenhaal, and John Mayer, who told Rolling Stone he was "humiliated" by Swift's "Dear John," which has her singing "Don't you think I was too young to be messed with?"