Mysterious black stains on wooden cutting board
The stain was probably caused by water and iron staining the wood. What you can do is get some OXALIC ACID powder (be careful this is strong stuff). Make a strong solution, blot it onto the stain, and leave a cloth soaked in the solution on the stain for a few hours.
The stains won't affect the quality of the board, or affect the food in any way.
Rinse the board with a soluton of water and vinegar.
Oxalic acid can be somewhat dangerous, do not get it near your eyes or inhale
any of the powder. It's sold as "wood bleach" at your local well-stocked hardware
store (but not, apparently, at Home Depot). There's a second substance sold
labeled as "wood bleach" which is a 2-part peroxide mixture. That's not what
you want here, costs much more (a lifetime supply of oxalic acid powder is
about $5), and doesn't work nearly as well.
Make sure you rinse the board very, very well after the acid treatment. It's
water soluble so a good rinse will remove it all. I get these stains on one of my
boards pretty regularly. I don't think it's rust because oxalic acid is completely
ineffective against rust stains but it cleans these things off in seconds. The
stains seem to be directly related to cutting potatoes.
Sanding the board is always a good idea. Makes it all new again. Hit it with a
sheet of 110 grit sandpaper wrapped around a sanding block followed by 220
grit and you should have a nice rejuvinated surface. Expect to spent 15-20 minutes
of reasonably hard work.
After destaining and sanding, you should give it a light oiling. Either some mineral oil,
'butcher block oil" (which is just mineral oil at 2x the price), or walnut oil (right
from the grocery store) would work. Don't use other sorts of vegetable, olive, etc oils.
These don't ever dry and can eventually get rancid. And *never* use non-food-grade
wood finishes or oils; they often contain all sorts of scary drying agents and stuff.