You wash the floors daily but the grout is always black, why?
Typically, it's the way you are cleaning. Do you take a mop and a bucket with soapy water to clean your floors? The soap that is cleaning the floor is also adding dirt to the floor. How you say? Let's start with the purpose of soap. Soap is designed to attract dirt. When you wash the floor and fail to rinse the soap off, you are attracting dirt to your floor.
Now let's look at the mop. The mop pushes all of the dirt and water off of the tile and deposits it in the grout lines. Grout is porous and soaks up the dirty water and voila, dirty grout. Consider using a steam mop which helps prevent the dirt from pooling in the grout lines. The microfiber mop head grabs all of the dirt and pulls it off of the floor. Also, you clean with steam rather than soap and you do not need to rinse! If you prefer to use a mop, consider using a clean water rinse to get all of the soap off of the floor.
Consider having your floors deep cleaned annually to keep your grout lines looking like new. Waiting too long will increase the soil load and may permanently discolor the grout. A professional deep clean or restoration will sanitize those floors and brighten those grout lines. All dirt and grime is removed from the house leaving your floors showing it's natural beauty. Grout sealing will help keep the dirt out between cleanings.