I can't find the plate... but see in many search engines (internet) that RGK is in the 'Bergman' book. And yes it uses black chenille.
I learned to tie trout flies, taught by a mountain man hermit, on 6xl size 8 hooks, by hand. The only fly I tied for then, beginning at around age 6, were 'wooly worms. Brown chenille, brown hackle, black button hole thread. I caught many many fish. Enough to where 'catching' was no longer important by the time I was an adult. I bought an inexpensive vise (sunrise), I never knew that flies were tied with the use of a vise until that age, after purchasing a book by Eric Leiser. I began tying flies for sale to a major sporting goods store and tied thousands of 'wooly worms' (and muddler minnows and and and) I half promised myself at that time that I would never use chenille again, if I could avoid it. I rarely use it today.
It's just a personal idiosyncrasy. I think ostrich makes for a better 'moving' fly in the water. The first flies I tied, replacing the chenille, were with feathers from a hotel maid's feather duster that was on her cleaning cart. The duster being made up of a bunch of natural brown, black and light colored ostrich herl. The flies I tied with those duster feathers were RGKs with cheap yellow hackle, ostrich herl and white buck tail (I got from a local taxidermist) wing. I was very proud of each trout I caught and released with that wet fly. As proud as catching them on a glass fly rod I built, to which was mounted a pawn shop Pflueger reel (the only reel I owned).
Just a little history about me. Have a fine weekend my friends. Spring is here !!!
ps, I didn't make much profit tying for the sporting goods store. Many of the materials I used to tie were the 'trimmings' from that local taxidermist and... he would often call if he received an animal that was too 'ripe', when brought to him, for him to work with. Including many legally taken birds. Ducks, turkey, pheasant, other range birds. Deer, elk, bear, mnt sheep and goats and etc. These materials made up the bulk of my supplies. (along with 'roadkill'. I doubt that today, one could sell flies to a store that were tied from those wild and natural materials. (having NOT gone through some sort of process to make sure they are disease free).