This song was written by Taylor Swift because Taylor Swift once went out with a guy and something happened between them and she got mad and they broke up. But than later, Katy Perry went out with the same guy and something happened between them and Katy Perry got mad and they broke up. So, that is what bad blood is about. Also, "Blank Space" was first to happen. The same guy came to meet Taylor Swift and that was bad. They were more and more better friend`s as time passed by. And then, the guy did something and they broke up. The very, exact, same, thing happened to Katy Perry and the very mean guy. After that, Taylor Swift, sang I Knew You Were Trouble. John Mayer is the name of the guy whom dated both extraordinary pop singer's. So strictly speaking, YES! It was a bit of everyone's fault {Although, since boys are lower on intelligence [only some] mostly it's John Mayer!} And all of these words that I wrote + Katy Perry Taylor Swift and John Mayer = Bad Blood. And one last thing, Katy Perry's "Dark Horse". It brings all 3 pop singers together in the ducky of awesome parody on YouTube. It's really funny and I really recommend it.