After normal use, wash the board with a stiff brush and running water. Then wipe dry and leave the wooden board to fully dry by a window so UV light can get to it
Once every few months (depending how often you use it) weight the board down with something heavy (glass container etc.) into a strong brine solution for an overnight soak. Remember to dry and re-oil with your favourite wood oil (cooking oil may promote black mould growth) after this treatment. There are many opinions on what a good wood oil is. See separate discussions
Once you have the black mould stains, they are hard to remove without physical damage to the board. The simplest treatment is to send the board off to a wood worker to run through a "thicknesser" machine (high speed wood plane), and a manual plane on the edges
Do not sand wood that is not going to be painted. Sanding opens up all the ends of the wood fibres, and makes then soft, and absorbent, which will result in you board wearing out fast, and it absorbing more contaminants
If using mineral oil make sure it is food grade