Use a poultice of hydrogen peroxide and baking powder. Mix and make it the consistency of a runny paste. 1st slather on a tiny bit in a corner, let it dry to a cakey crust overnight and remove. Make sure it didn't do any harm. If it didn't, repeat and place on stained area. Close area off as it is not good for pets.

Afterwards, seal if you haven't already or reseal if its been awhile,

Thanks so much - will try tonight and post with an update!

Shouldn't it be baking SODA not powder? Or maybe it doesn't make a difference?

Out of curiosity, did this ever get resolved? I am having a similar issue with Carrara marble. Housekeeper accidentally used Kaboom on our shower floor. Blue spots all over. I used a poultice and I thought that had solved the issue - no more blue. BUT 24 hours later, the blue is back...

Google Fred Hueston's "Blue B Gone". Braxton Bragg maybe?