It does look like breast blister to me, which is like a pressure sore. If she's laying down a lot, that's what can happen.
@casportpony has a short tutorial about how to clean it out in this thread. Chicken pus does have to be removed, it will not dry up on its own. Once cleaned out, chances are better at it healing and closing up.
https://www.backyardchickens.com/th...ister-graphic-pictures.1096276/#post-16850510
Have you tried propping her on rolled up towels to help get the pressure off her breast?
If she can't stand for long periods of time, then a sling may be helpful. She can stand, but be supported by the sling.
Alternating her positions can help with healing too. It would be similar to having a patient in bed the majority of the time, you have to keep moving and turning them to help prevent sores.

I'm sorry to hear about Clyde, I'm sure Bonnie and you both miss him. At 3 1/2 years, you've done a good job taking care of them. Bonnie looks like a sweet girl.