Things You'll Need
Stiff scrub brush
Dark spots on grout are most likely the result of mold, mildew or plain old dirt buildup. The joints of many tiled surfaces are filled in with grout and, just like the tile itself, grout is susceptible to dirt and grime. Furthermore, grout is often exposed to moisture, which makes it an ideal condition for mold and mildew growth. Whether the grout is dark from mold, mildew, dirt or some other substance, removing dark spots will keep the grout and the entire tiled surface looking neat and clean.
Use a dull knife to scrape away as much of the dark spot as you can. If it is already embedded into the grout, you will likely not remove much. Be careful when doing this so as not to chip or damage the grout.
Mix a cleaning solution into a bucket. Combine one part dish soap and four parts water.
Dip a stiff scrub brush into the cleaning solution. Scrub the dark areas of the grout with the cleaning solution and scrub brush. Apply some pressure and scrub briskly.
Mop over the area with plain water. Allow the grout to air-dry and inspect for any trace of the dark spots. If the spots were mild and more on the surface of the grout, the soapy water may have done the trick. If the spots were more embedded into the grout, a stronger product may be needed.
Pour some undiluted hydrogen peroxide into a spray bottle if the dark spots persist. Spray the dark areas of the grout with the peroxide.
Allow the peroxide to penetrate the grout without rinsing. Scrub the grout with an old toothbrush to loosen up the dark spots. The peroxide will evaporate within several hours and, hopefully, the dark spots will be gone as well.
Use oxygen bleach as an alternative method for cleaning dark spots from grout, or if the spots were not removed with the above products. Mix a solution of oxygen bleach and water into a bucket. The exact ratio of oxygen bleach to water will vary depending on the concentration of sodium percarbonate in the product you are using. Refer to the label for specific mixing instructions.
Pour the oxygen bleach solution directly from the bucket onto the dark spots. Allow it to sit for at least 30 minutes.
Scrub the grout with a stiff brush after the appropriate time has elapsed. Rinse with a mop and plain water.
Test the product you are using on an inconspicuous area of the grout before applying it throughout. Be sure that the product does not discolor the grout.