Don't blame yourself, clearly the rescue was trying to hide how sick she was. You should put them on blast everywhere with the name of the rescue- especially on the Fauna Classifieds BOI. That way other people will avoid this sham of a rescue.
I'm terribly sorry for your loss. Don't give up on keeping snakes, you sound like a fantastic keeper so don't let this bad experience ruin it for you. Just for next time, be sure to thoroughly examine the reptile. If a rescue/breeder won't let you hold a potential purchase that's definitely a red flag.