I've been comparing the two species on BugGuide and in Bumble Bees of North America. A lot depends of whether this bee is a female. I think it might be because it seems to have pollen on the hind leg. Female appositus have a black pleura, as does this bee, while female B. sylvicola have a more yellow pleura. Also, the upper hair on the scutellum is whitish, where the lower hair is more yellow, typical of some B. appositus. The brownish rusty hair on the abdomen extends to T5. The other thing that bothered me was the dark charcoal wings. B. sylvacola wings are lighter. And, B. sylvicola is rare below 10,000 feet in our area. Our individual was photographed at 9240 feet.
As I was comparing individuals on BugGuide, I found an individual that is very similar to this image, #429723, B. appositus from Arvada CO. It has the same coloration, the black pleura, the charcoal wings (thought not quite as dark as our image).

I suppose it must be appositus

Thanks so much for this ID, Dr. Ascher. This bumble bee has had us puzzled. Your help is much appreciated.