There exist (2) types of wood bleach that I know of. The first and mildest is Oxallic acid sp? which will get out what I would consider organic stains like the black rings that occur under potted plants on grandma's antique table. It comes as a powder or crystals which need to be mixed with warm water. After the stains disappear (hopefully) you need to nutralize with white vinager, let dry then sand. The dust is fairly irritating to the sinus so use a mask.

The other bleach is a 2-part liquid mix that will get out things like ink. Be careful though because this stuff will take out not only the stain but the natural color of the wood also. Usually you need to add color back when you are done (unless you like a pallid grey-green).

Be sure to wear an approved dust respirator, not something labelled as a dust mask. An approved dust respirator will have an approval from NIOSH (National Institute for Occupational Safety and Health) a government agency. A mask with an N95 approval should be sufficient for low concentrations.

Good luck on your project!