Black stains on shower caulking are often attributed to the growth of mould in your shower. Mold thrives in moist, warm places such as shower stalls and quickly spreads to tile as well as caulk. Some types of mould are toxic and can be dangerous to your health. Chemical products are available to remove mould and other black stains from your shower caulk, but you can remove them yourself using supplies from home and without exposure to harmful chemicals.
Create a paste of equal parts lemon juice and table salt in a bowl. Dip an old toothbrush into the lemon juice and salt mixture and scrub it into the black stains on your shower caulk. Allow the lemon juice paste to sit on the black stains for 15 minutes to kill mould and loosen and lift black stains.
Wet the old toothbrush and give the black stains on your shower caulk another scrub. Rinse with plain, cool water.
Tackle any remaining black stains on your shower caulk with straight white vinegar. Pour vinegar into a spray bottle and spray directly on the black stain. Leave the vinegar on the black stain until the vinegar is completely dry.
Dampen a clean cloth with plain water. Wipe away last traces of black stain on your shower caulk. Rinse the cloth, if needed, and continue to wipe until the stain is gone.
Dry your shower caulk with a clean, dry cloth.
Add 1 to 2 tbsps. of baking soda to your vinegar to make a mild abrasive for cleaning the black stains.
If stains persist, they may be black toxic mould. Consult a mould removal specialist as your next course of action.