Definitely cut down on the amount fed. Also, I know of many people who have had problems with Blue like you've experienced - I've never had good luck with it myself. You might want to try another brand, such as Taste of the Wild or Fromm. Some dogs have a problem with high protein/fat, so you could look for one with a lower percentage of each. I had a boxer who couldn't eat more than 25% protein/15% fat (higher % gave him very loose stools), so he did best on Taste of the Wild Pacific Stream (found at private pet stores and Tractor Supply; some feed stores). Nutrisca is also easier on the tummy for many dogs.
Try transitioning slower. For example, 1/4 new, 3/4 old for a week, then 1/2 new/old for a week, then 3/4 new, 1/4 old for a week, then total new. Some dogs need to transition even slower than this.
The dog will be going through a sort of detox - sort of like a person going from a complete McDonald's diet to a healthy one.