So sorry for your loss, my heart goes out to you....poor Morph, I can only imagine how devastated you must be feeling
When one of my dogs was attacked at Xmas we rushed her home without getting the owners details as had to get her out of that situation quickly, she was on a lead. I managed to track the owner down last month and let them know the extent of the damage caused and the huge vet bills and are currently claming though their insurance. But more importantly I wanted to track them down as they weren't away that their dog had even bitten mine as they had gone off ahead of their dog when it happened. If you can I think you should track down this owner and make them aware of what happened to poor Morph!
The police will do something if a dog kills another under the dangerous dog act.
As a rule I don't allow either of mine to socialise with dogs they don't know, I don't allow strange dogs to even sniff them as you can never tell whether two dogs will get on or not. Only yesterday a dog (not a bull breed) ran over off lead aggresively brandishing it's teeth and went straight for my dog snapping for no reason other than it obvioulsy doesn't like the look of her, I got in the middle and had to drag her off the ground by her lead and swing her behind me to protect her.
I hope the owner of Blue is told about this and will seek the help of a behaviourist, keep Blue on a lead at all times and muzzled from now!