Hi Morgaine,
I might be wrong, but I think that's dead tissue caused by impaction of the breast muscle against the sternum (breastbone).
It happens sometimes with large cornish x if they too much wing-flapping exercise. Their breast muscle bulk is so huge the muscle closest to the breastbone can compress and die off. It can be a big problem if they're kept slightly older than recommended dispatch time.
Nasty looking, but I've kept carcases like this, making sure I remove any discolored flesh.
I may be wrong but that's what it looks like to me.
cheers
Erica