Hi, Margot. You probably should treat it like any stain but you will have to do a trial first to see what works. I think that I would start with a trial of a paint stripper like Citristrip. Glop some on one board. Let it sit a couple of hours and take it off with a putty knife and stripping pad, then wash it off. This may work to some degree.
When it has dried a little, sponge some household bleach on the trial area then rinse it off with water. If this does not work, then mix up some Wood Bleach (oxalic acid and available at home centers) and try it on another section of the wood that you have stripped. Neutralize it with one part vinegar to five parts water. One of these should work but it is impossible to tell which one ahead of time because of the different formulations that could be used for the stain.
Please handle all these chemicals with care and wear gloves and eye protection when using them. Wear a dust mask when you are mixing the Wood Bleach.