How to remove black from floor joints

Pour baking soda into the floor grout lines.
Add hydrogen peroxide and liquid dish soap.
Leave it on for 5 minutes, then scrub well with a toothbrush and rinse.
White vinegar

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White vinegar can be used in multiple ways. In addition to cleaning joints well, it is effective against lime and mold.

Pour vinegar on the joints.
Add a little baking soda.
Scrub vigorously with a toothbrush and then rinse.