I am so terribly sorry for you all. Something was obviously very, very wrong with your dog that went undetected until it was far too late. There are many things that can show no symptoms and result in no cause for alarm until death occurs. The same thing is true for humans. Sadly, our bodies aren't always in perfect working order.
Without a post-mortem exam (autopsy) you will never know exactly what happened. We all want to know in cases like this. It's like being granted absolution in that somehow having our helplessness confirmed makes the death easier to bear. It was NOT your fault. You did nothing wrong. The vet did nothing wrong.
It's a hard thing knowing that our animal companions will never live as long as we do. Some day we have to say goodbye. What helps me through the grief is focusing on the great lives my critters had. The end and what caused it are of little importance. It's the love, care and companionship that came before the death that are important and meaningful. :-)
Welcome to the dog forum, you know,,,,I couldn't not be forsure...but, I have seen people here post to the vet forum...over to right of this screen, see, "ask a vet" Dr. Cheng, and the vet response w/very informative info...
Blood work only showed dehydration? Hum, maybe your dog had a heart problem you were unaware of, enlargement,,labored breathing onset is quick and death is quick..
Very sorry for the loss of your pet, I know our beautiful night time skies are loaded w/twinkling stars that respresent all the deceased pets...so, now theres another star with golden's name who is gaurding the heavens to await your arrival on day, some call it rainbow bridge....maybe one day in the future your love for your golden can love again another pet who in need of a pet owner like you full of compasion and w/heart...