bald breast

Sounds like a partial molt so nothing to worry about.  You have checked for everything else so it is a matter of waiting for her new feathers which sound as if they are coming through. 

I have one the same and also a Black Rock with a bald bottom since molting in the Spring.  Either she or one of the others is pecking her new feathers before they get chance to grow.  I have tried purple spray and also zinc and castor cream but neither made any difference.  Talking to other hen owners this seems to be a common thing. 

The only other thing I thought of trying was a sports bandage to cover her while they regrow.  The thought of trying to wrap her up leaving her vent uncovered so she can still poo has stopped me trying this.  I might pluck up the courage (pardon the pun!) to giving it a go if her bottom is still baldy for the autumn.