I don’t think I’ve ever seen a Cairn that dark, even at shows (been ages). Maybe it’s just the lighting!
There was a link with small swatches of coat colors and the dark brindle did look that dark. It may not be a popular or preferred color. I don’t see them very often, and I have to admit when I think of cairn, Toto pops into my head first. I’ve seen that darned movie too many times. :)