The key to keeping the grout line as black as possible will be the clean up during the honing process. Hone small areas make sure you gave neutral cleaner in your honing water the help keep the slurry suspended.
After the honing process you may want to use your spinner tool for the finial clean up. If the grout is non sanded you should not have too much of a problem.
John E. Freitag
The Stone and Tile School
Next week we will be honing dark gray limestone with black grout lines and I must keep the grout as black as possible. We will finish with color enhancer but I can't tell if that will enhance the grout color to the original black. I was thinking about adding some kind of dye to the honing powder. Is this a good idea? What do you recommend and where can I get it?