If your grouting has gone dark or black, it is just dirty. 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. The dirt is washed off the tiles and collects in the grout joints. It settles in these mini gutters and dries out leaving a film of dirt and detergent. Each time the floor is washed, this film builds up and eventually causes the grout to appear black. In truth, the black is just the film of grime sitting on top of the grout.
In some areas where there is ongoing dampness, such as a bathroom, this film is an ideal breeding ground for mould.
Cleaning the floor with too much water and detergent and then not rinsing thoroughly is the most common cause. We should explain here that the way detergent works is to capture dirt and grease on the microscopic level. The reason we mix it with water is to enable this mini globules to be washed away. If we leave a floor wet without rinsing, that film will stay on the floor, especially in the grout joints.
Whenever you wash a floor using any form of detergent, rinse it well with clean water. Most soiling on glazed ceramic tiles will lift off with plain, clean, warm water. The only thing we recommend that you add to the water is a drop or two of disinfectant (or a splash of vinegar) to kill any germs. Any greasy or oily marks can be cleaned up as a spill. For overall cleaning, leave the detergent in the cupboard.
purchase a bottle of Heavy Duty Tile and Grout Cleaner. Wash the floor (and rinse several times) to remove all the built up grime.
From that time on, avoid detergents and just use warm water. Always use the water sparingly; there is no need to flood the floor.
Once the floor is thoroughly clean and dry, it might be wise to seal the grout joints with Grout Sealer. Onle light application using the trigger spray will seal the grout joints and make them water repellant and easy to clean for around five years.