Ah maybe, hadn't really noticed that. I guess it might depend on tone but could be a risk - my thinking is just that the grout lines might be quite bold if in solid black with the KitKat tiles meaning lots of lines
Are you doing the tiling yourself? When my bathroom was tiled, the tiler brought out a sample pack of grout colours that made the decision much easier as could see the actual colour it would be?
Actual black black looks like too much contrast, what about mid / dark grey for similar effect but more subtle?