One jelly bean from the jar labeled red/blue will do the trick. The key here is to realize that each jar is mislabeled. The red-jelly-bean jar is not actually the red-jelly-bean jar, and the same goes for the other jars. Taking a jelly bean from the jar labeled red/blue, the mix jar, is the quickest way to solve the problem. Once you choose a jelly bean from there, you will know for certain whether that jar contains red or blue jelly beans (each jar is mislabeled, so this jar can't be a mix of both colors).

Let's say that you pull a red jelly bean from the jar labeled red/blue. Now, you know that this jar contains red jelly beans. There are two remaining jars: one labeled blue and the other labeled red. Because each jar is mislabeled, and you already know which jar contains red jelly beans, the jar labeled blue must be the mix jar, and the jar labeled red actually holds blue jelly beans.