Wooden cutting board turns black

The black spots can be one of several things, but are likely a variety of mold. Black stains can also be caused by a reaction to the iron in your knives (particularly if you use carbon steel blades instead of stainless steel). If it is mold, it is growing because even though the surface of your cutting board is dry, moisture has soaked in, probably because it needs to be oiled or waxed.

To recondition a butcher block or cutting board that is stained or gouged, you can sand the surface down, and reoil.

1. Use a sanding block to keep the surface from getting wavy as you sand.
2. Start sanding with 120 grit (80 if you are removing big gouges), and sand evenly, moving with the grain, until the stains are gone, or almost gone (if a few very deep ones survive, don't worry about it).
3. Repeat the sanding with 240, then 400 grit until the roughness from the previous sanding is gone.
4. Reoil with any Butcher Block Conditioner. There are many out there, and they are either food-safe mineral oil, or that oil with a few other waxes mixed in. Many are good, but my favorite is Howard's Butcher Block Conditioner. Follow the instructions, but it will amount to making a liberal application, allowing it to sit for a while, then wiping off the excess.
5. Maintain the new surface by drying the cutting board well before storing (as you are already doing), and wiping lightly with mineral oil briefly every few times you use it, much like maintaining a cast iron pan.

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

I agree with the statement made by Sam that Howard Butcher Block Conditioner is the best thing for Butcher Blocks and cutting boards by far. My suggestion is that you use Howard Clean-A-Finish Wood Soap to get rid of black marks (or any marks) by spraying it liberally onto the surface then scrub into it using a 3M type abrasive saturated in water until those marks disappear. When it is completely dry you can then sand it smooth with 400 grit abrasiv. When that is done you re-condition the wood with a coat or two of Howard Butcher Block Conditioner leaving about 15 minutes between each coat before wiping excess conditioner away with a clean cloth. Each time you wash you board after using it, make sure that you oil it with Howard Chopping Block oil before putting it back into storage. By doing this you'll have bacteria free boards to work with every time.

The simplest way to prevent it from happening is to never clean your board IN water. You should always clean and rinse your board only with a wet cloth, then dry immediately with a towel and store standing on one side, instead of flat.

If you use more water than this your board will also get deformed and cracked much sooner. Water makes wood expand, so a wet-dry cycle is very bad for a wooden board.

You don't need any products to solve this issue.