If the lines between your tiles have gone dark or black, it is just dirty grout. This does not mean that you don't keep a clean home or floor, but simply that the way you have been cleaning is not best suited to tiles. The most common cause of this is that people wash their floors using too much water and detergent.

Why is my grout turning black?

If your grout has turned dark or black, it is likely just dirt. ... Tile grout can also turn black due to mold. Mold usually develops in areas where there is high humidity or in areas with moisture. Mold is commonly found on bathroom shower tiles or floors and also on kitchen backsplashes.

How do you clean white grout that has turned black?

Whiten Grout Using Vinegar

Fill a spray bottle three-quarters full with undiluted, distilled white vinegar. Spray the stained grout with the vinegar. Let the vinegar soak in for five to 10 minutes so it penetrates the grout. Scrub the grout with an old toothbrush or scrub brush to lift the stains.