I haven't used anything, but I love the know as well. I have a black/bay and I'm dreading the fading.
But there are plenty of color enhancing shampoos (and supplements):
Sunflower SUNSCREEN™ RTU
Black Pearl Shampoo
Jeffers Black Coat Shampoo
Hi Black Burgundy
The first two are what I think you are looking for.
I haven't used them (except Quic Black and Quic Color), but theses are things I looked up. I'm curious to see if anyone else has. Also, there are plenty of supplements that will help prevent a dark horse from bleaching out, I use Black as Knight.