Cutting board mold
Another possibility is a reaction with the iron in the knives you're using. I've seen this
with maple and oak, but have no idea about bamboo. With maple and oak, the thing to
do is bleach it out. I usually use oxalic acid, which is sold as "wood bleach" in the
finishing department of your local well-stocked hardware store. Follow instructions,
rinse well. Potatoes sometimes leave black stains behind, also.
Since it's black, it's likely mildew which is a kind of mold that's particularly hard to get rid of once it starts. It may be harder to get out of bamboo (which is a grass) than it would have been from a wood cutting board which tends to have natural bacteria-fighting properties. I get this periodically on wooden items in a house that I close up for the winter and it's a real nuisance.
Use a solution of regular household bleach in water. You can use as much as 1 cup to a gallon of water, which is what we used when we were cleaning mildew after Katrina in New Orleans. Scrub the board with the solution and a stiff brush. Rinse well.
Sunshine is the best friend you have. Place your cutting board outside in full sunlight for several days. You may need to use the bleach solution several times. If the mildew comes back, do it again.
Don't store the board in an enclosed place. Even if it feels dry, it may have residual moisture and any mildew spores will thrive, causing the problem to start all over again. Clean your board regularly with a mild bleach/water solution to prevent further problems. Once you do the initial cleaning, a solution of plain white vinegar and water is just as effective as bleach for routine cleaning if you object to the odor of bleach. The smell of either will disapate in a short time however and both are safe for food surfaces.
Well, if you left food residue on the board & it was damp and it was put away in a cabinet, then it could have easily molded.
Personally, I would use straight bleach on the board & let it sit for 10 min. The rinse off well & dry with a towel. If you have a disk rack, let it air dry for 2 days before putting it away. That said, I always leave my cutting board out in the air to dry & never put it away. Sometimes I leave it on a stove burner for good air circulation.
In the future, use a mild bleach solution to wipe it down with.