Bleach does nothing more than kill mold , it does not "bleach out"
limestone or change its color from when it was cut. You said the sponge
"Bleaced out" the area, well all it did was clean and kill mold, nothing
more harmfull than that. Stone is porus and remains wet, mold grows
where conditions are right, like in your shower. So go get your laundry
bleach and pour some on, you have thought this out to death and the
answer is simple, Bleach.