I would suspect you switched her too quickly. Sometimes they are soooo sensitive you have to literally count kibbles and add only a few more at a time. Silly little babies.
If she continues to have diarhea I would get her seen by the vet. If she checks out clean I would switch food. I am not sure how long she's been getting the Blue Buffalo but it could be a transition-thing that will get better. I would be pretty worried about the blood in the stool and probably would look for something else though. I am super paranoid
though so that's just me!