The Blue Grouse was split into the Sooty Grouse and the Dusky Grouse in 2006. Before that, they were considered differently colored subspecies of the Blue Grouse. The Sooty Grouse lives on the Pacific Coast and has darker plumage and a yellow vocal sac. The Dusky Grouse lives in the Rocky Mountains and has a red vocal sack. You can clearly see the yellow vocal sack in some of these pictures.
The Sooty Grouse is frequently heard and rarely seen. The low "whomfing" of the grouse can be heard over fairly good distance, but it is extremely difficult to echolocate on this low sound. So finding the grouse can be very hard.
The pictures below are all of the same male grouse on the tail up to May Lake.