Clean your grout by slowly poring a thin layer of oxygen bleach solution until you cover the entire area that you wish to change color. It will remove the dirt and grime from the grout. Refer to the product's label for the minimum cure time required.
Purchase a 25-ounce bottle of grout additive from a home improvement store. Look for features that help the grout become stronger and more stain resistant. Dip a thin paint brush into the bottle and carefully brush the gray grout area until it is completely covered.
Wait for the additive to dry for several hours depending on the properties of the specific brand that you purchased.
Dip another thin brush into an oil-based grout stain. It should be the bold black color that you are looking for. If possible, compare the color to samples of the black shade that you want. Slowly brush the gray grout until it is a deep, rich black color. Let the stain dry for several hours according to the specific requirements of the product you purchased.