Sometimes, dark spots are the result of grease buildup from years of doing laundry or using liquid fabric softener. Many retailers now sell products specifically designed to clean washing machine drums, but running a cup of white vinegar or bleach through the machine on an empty cycle will also do the trick.
In this manner, how do you get brown stains out of clothes that have been stored?
Rub a paste of baking soda and water with an old toothbrush onto the stain. Allow it to sit overnight. Rinse and repeat if necessary.