First of all, I'm very very sorry. What a horrible experience!!

My thought was that the Russian Blue might have picked up something when it got lost outside, but I don't know of a virus that's acting that agressive and fast (unless it's the distemper virus, but those are different symptoms).

Maybe it could be this new form of the calico virus that occured in parts of the United States lately. The dark urine seems to point out that the liver might have been affected. Does anyone know if that new calico virus causes liver failure or infection?

However, poison could be another option. Did your parents use any toxic subtances (such as anti freezer) in the house lately, or maybe brought a toxic plant home?

I'm sorry that you didn't have the chance to bring them to the vet!

Kirsten