Can you post some photos of the whole hen too?
When you were looking at the blisters on this one that's not well, did you happen to notice if any maggots were underneath her poopy feathers?
This time of year FlyStrike can occur and maggots can hatch within 24 hrs or less - the maggots begin to consume healthy and compromised flesh alike. They can kill a chicken in a couple of days if they are not removed.
I couldn't say what's causing the blister like areas - maybe something is biting or stinging the hen? Ants? Spider? Do you notice any bugs crawling around in the feathers?
I'm sorry you lost one, hopefully you can separate this one out for observation and be able to figure out what's happening. I would give her a good wash up, use a gentle soap.