Can cortisone injections into keloid scar from breast aug cause skin to thin to the point where implant can break through skin?

The bottom half of your implant is only covered with skin and a small amount of fat/breast tissue. It sounds like you are at risk of implant exposure. I would get surgery soon otherwise you may lose the implant. In the meantime I’d advise wearing a soft supportive sports bra to take tension off the skin.

Yes, cortisone can definitely lead to thinning of the skin and if it appears blue it usually means the implant is just under the skin. When an implant is placed "under the muscle" this means that the upper half of the implant is under the muscle but the bottom half is not, because the pectoralis muscle stops at about the level of the nipple. If an implant breaks through the skin often it is necessary to remove it and wait six months before replacing it, so it is often a good idea to be proactive if the implant is very close to the surface. Best wishes.

Side effects of using cortisone injections include thinning or sunken skin at the injection site.  Steroids are known to break the bonds between collagen fibers, which reduces the amount of scar tissue beneath the skin. Your PS has the benefit of seeing you in person so I would follow her advice regarding removal of the thin skin.