Dip an old but clean toothbrush in warm water. Then dip the dampened toothbrush into a granulated oxalic-acid based cleaner.
Rub the toothbrush over the stain, loosening it. Allow it to penetrate for two or three minutes. Repeat with more cleanser if necessary.
Fill a spray bottle with cold water and spritz the stain heavily. Wipe it off with a lint-free cloth and repeat, rinsing the area.
Dip a cotton swab in rubbing alcohol and pat down the dark spot until you saturate it well.
Blot the area with fresh pieces of two-ply paper towel. Repeat the entire process again and evaluate the area. If the stain isn't gone or still appears faintly, go on to step three.
Hold a can of hairspray 6 inches away from the dark spot and mist it lightly. Allow it to penetrate for 10 seconds and then blot it with a fresh paper towel, if the item is fabric-based. If it's metal, plastic or wood, wipe off the hairspray.