Blue buffalo is not a terrible food, but in my opinion, paying more for a food does not mean it will be better for your dog or that it is full of better ingredients.
Generally, food allergies are fairly rare in dogs, contrary to what people seem to believe. This certainly doesn't mean that it's impossible for dogs to be allergic to food, but I tend to like to rule out everything else first.
Dealing with itchy dogs can be tough, but the first thing I would do is bring him to the vet for a skin scraping. Your dog could have developed an allergy to fleas (meaning you probably will not see any fleas on the dog...allergic dogs tend to have less parasites interestingly enough), he could have developed a skin infection (often this can mean there is something else going on systemically, so it's good to rule out anything else - Addison's or cushings disease for example), or it could even be the start of some mange (again may point to something else going no on as well). Your vet may decide to try some prednisone to see what the response is. If no response and the skin scraping comes back clear (and a round of antibiotics does nothing to help), they may want you to try a food trial. That is really the only way to truly know if a dog has allergies, and even then it's tough to totally restrict their diet.
It's not a terrible thing to change the diet and see if it helps, but I would not be disappointed if it doesn't have the results you expect. I would definitely recommend a trip to the vet for some routine blood work, physical, and a skin scraping to rule out any other medical causes.