Moisture Damage on a Hardwood Floor?

Today I found a large black wet stain under a plant on my hardwood oak floor. I had placed a bowl under the plant, but the water must have gone over the top of the bowl and has sat there for a few days.

I have cleaned and dried the water mark with dry cloths and removed some of the stain with a weak solution of bleach. Some of the black mark came up, but some turned green. My question now is, should I let the stain dry and try sanding it down or try another bleach treatment?

By Joan Wilson from State College, PA