I don't know the story behind it, but one of those saddles has been on my Riv custom for more than 20 years. As for the grey being exclusive to Rivendell, I also have a grey Brooks Pro, which definitely didn't come from them - I bought it down the road from a local bike shop.
In any case, what you will find, if you use the saddle, is that the grey is an additional layer of dye over the normal black, and after some use, the the grey wears off on the higher wear areas, showing the black underneath. Here is what my grey B17 looks like these days:
Nick Payne