You did get the lightest gray ink in Clark's
Sampler 
.
You could reserve it for dip-pen use, since dip pens write wetter (and therefore darker) than most FPs. A wet, broad FP or dip pen might make dark enough marks for you.
I haven't heard of problems using J. Herbin inks in mixes, so maybe you could create experimental mixes with whatever black ink(s) you have.
The only other factory-mixed, neutral-hued gray in the book is Sailor Jentle Gray, and it is darker. All the other grays in the book have a very slight greenish hue.
The book does not contain Diamine Gray, which appears neutral and dark in the digital images I've seen.