1 of 2:
Oh wow this sounds like our baby. She is a lab/Great Dane mix with Shar Pei thrown in. We started her on Blue Buffalo when we got her at 12 weeks and she had nothing but soft stools. Also went through testing after two nights of her having explosive, uncontrollable diarrhea. She was put on prescription Science diet I/D which was FABULOUS but ridiculously expensive and a pain to get b/c it has to be at the vet. When we tried weening her off and mixing it with the dry food, the stools went back to the same "frozen yogurt" consistency (husband's analogy). Tried chicken and rice mixed with kibble, still soft. Back on the I/D, fine. Vet said that a lot of pups have problems with the high protein in Blue Buffalo and have similar issues.
What we ended up doing was going to Pet Valu store, a Canadian based company just starting to pop up more in the US, and talking to them. All of their staff are trained on all of the foods that they sell to know what's in it and what is best for each issue you may have. Granted, some employees DEFINITELY better than others, but we ended up switching to their brand of food - Performatrin. It's definitely more expensive but so far has been WELL worth it. From what I've read/heard Canada has much stricter regulations on what goes in their pet food, and the Performatrin brand has lower grain. Within a week of switching to the Performatrin, no problems. Stool hard as a rock. ETA: this is what we buy her http://www.petvalu.com/dog/food/product/10401/grain-free-recipe-performatrin-ultra