I am so sorry to hear this. When they did the blood work, did they check for Leptospires? It's caused two healthy dogs here to go into kidney failure. They catch it from contaminated water/urine from infected animals. In our area, it was rats that caused it. But it could be other dogs, deer, cows, racoons.. etc.
They probably tested for it, but there's nothing else I can think of that may cause this so suddenly.