Is there a way to 'remove' blackened water-stain from hardwood floor?
Removing Dark Stains With Sandpaper
  
  
  Remove the finish over the stain gently with sandpaper, moving with the grain of the wood. Use #100-grit sandpaper, and then feather the
  edges with #150-grit sandpaper.
  Sand the stain with #150-grit sandpaper, now that you have removed the finish. Feather the edges around the stained area with #0000 steel
  wool.
  Use tack cloth (lint-free cloth) to remove sanding dust.
  Put on several light coats of varnish to match the original finish.
  Feather the edges of the new varnish with #0000 steel wool to remove the slight bump between the old and new varnish.
  Wax the wood with a quality polish.
  
  
  Remove Dark Stains With Bleach
  
  
  Bleach the wood with chlorine bleach if the stain turns out to be too deep to remove without excessive sanding.
  Don your rubber gloves and apply the bleach with a brush.
  Let it sit for a few hours. The stain should fade to nearly the wood’s original color, but it’s a slow process.
  Use a clean sponge and water to remove the bleach completely and prevent further fading of the wood color.
  Apply vinegar to neutralize the wood. This will prevent the wood from lightening the stain or varnish when you brush it on.
  Let the wood dry thoroughly.
  Apply wood stain, if needed, and let it dry again.
  Brush on several light coats of varnish to match the original finish.
  Feather the edges of the new varnish with #0000 steel wool to remove the slight bump between the old and new varnish. Remove dust
  with a tack cloth.
  Wax the wood with a quality polish.
  


Taken from: http://www.wikihow.com/Get-Water-Stains-Off-Wood