I'm a big fan of grey ink, Iroshizuku Fuyu Syogun being my favourite. I posted some swabs here
I can't give you any advice on dark greys, I know there is talk about watery blacks, some people have indicated that Parker Black and Waterman Black shade quite a lot, with the positives and negatives that that implies.
Rather than mix, you might also try diluting a black to get the colour that you're looking for. If so, I believe the nano-carbon / partical inks perform very well (Sandy1's sample of Sei-Boku - blue black - diluted)