Since you did your own work, you have some grout to experiment with external to your job. You might mix a little patch of grout and after it has set for a couple of days, put some sealer on it and see if that's dark enough. Seems unlikely you'll find grout colorant darker than raven black. Now here's my idea and yeah, I agree it sounds stupid, but since you don't have a great deal of grout to deal with, and you do have the option of experimentation....(drum roll....)how about doing your own coloration with black Sharpie pens. This worked as an ebony "stain" on a grand piano miniature I built...wonder what it would do for your grout. Luck, Kizmet.