Mildew stains on wood are often black but can also be other colors (such as red or green). No matter the color, mildew is unsightly and common on unfinished wood. The appearance alone is sufficient to warrant a good cleaning, no matter what you intend to do with the wood item.
In addition, if you plan on painting or otherwise finishing the wood, removing mildew is essential because the finish will not adhere well to the wood's surface. Scrubbing alone with a mild soap and water may help remove the stain, but mildew will likely return. To fully eliminate a mildew stain, you must also kill the mildew--not just remove the evidence.
How to Remove a Mildew Stain From Unfinished Wood
Dust the wood with a dry paint brush. Make sure to clean out crevices. If there is a lot of mildew and it is dry, you can also use a mildly abrasive sponge or sandpaper sand off the mildew stain.
Put on safety glasses and rubber gloves. Ensure that your area has adequate ventilation before you proceed to the next step.
Mix household (laundry) bleach with water in a clean bucket--1 cup to 1 gallon of water. Water can be cool or warm. Hot water more easily warps wood--avoid using hot water.
Wet a sponge in the bleach solution. Rub over the mildew stain, saturating it with the solution. Allow this to remain undisturbed on the wood
Dry the wood with a clean towel. Turn on a fan and aim it toward the wood. This will promote further drying (to prevent warping of the wood).