So you have Galaxy black on the floor? I'll find out for you mate my friends sell granite everything including tiles,its Sunday here Ill ring him tomorrow he is the granite man out here and Im the marble man. I would consider re grouting it in black first up, maybe a dye painted into the joints and wiped off the granite each time. Something that wont soak into the granite (you should be able to do that with the right product) and test on the edge of a spare tile.