It is very easy to dye the locks grey. It depends on how dark/light you want the locks. Black dye has a tendency to shift, because it is a mix of several colors. Start with a few drops and then add if you want it darker. I use kemtex, Dupont or food colors.
If you need a small amount it is best if you dye them yourself Useb black dye as wolgelukkig suggested. You may end up with some bluish tones since black in my experience is a rather tricky colour. I dye them dry, just soak them in your colour, heat up and let cool down then rinse well.