Remove any blackberries or blackberry fragments from the surface of the clothing by scraping them off with a spoon. Discard the removed berries.
Fill a bucket with cool water. Dunk the garment in and out of the water for 2 to 3 minutes to loosen the berry juice.
Empty the bucket and refill it with 1 quart of cool water. Add 1 tablespoon of white vinegar and 1/2 teaspoon of dish soap. Stir the mixture to combine all the ingredients.
Submerge the blackberry-stained garment in the vinegar and detergent solution. Allow it to soak for 15 minutes to loosen the blackberry stains.
Remove the garment from the vinegar solution. Wring out excess moisture. Lay the garment flat with the remaining berry stains facing up.
Wet a clean sponge with full-strength rubbing alcohol. Dab at the remaining blackberry stain until removed. Rinse the garment to wash away the stain and alcohol.
Launder your garment with heavy duty laundry detergent and oxygen bleach. Ensure the stain has been removed before placing the garment in the dryer. If any blackberry stain remains, repeat Steps 3 through 6 to remove it.