I am so sorry your dog went through this, you must be devastated to have lost him in this traumatic way.
If you want to take this further then I suggest you write to the Organization who deals with Veterinary Complaints in your Country. I’m in the UK so don’t know who it is, but I’m sure you will find them online.
You would need to send all the notes/invoicss from the Vet who dealt with him and also possibly a Report from the last Vet you took him to.
It is really not ethical for us to comment about this on here. However, I am very sad this has happened to your dog. My thoughts are with you.
UPDATE: It’s only been a few hours since my post, but the thing ruptured. It’s gone from the dark purple to a clear ring with slight red around it. I’m guessing it was just a large pimple or blister of some kind. I’ve cleaned and disinfected it. I’ll keep an eye on it, but I’m guessing she’s going to be just fine.
Keep it clean probably and infected pimple like bump. I use peroxide on my dogs.