Some Bombus hypnorum [tree bees] have a dark form [you did see some dark brown - rather than the tawny brown usual thorax colour] but they always have a white tail. I think that your bumble might be one of these dark ones. Did you see any other dark workers foraging around your garden?
Thanks, I think you’re right. No similar workers, but this was a new queen so no descendants yet. Plenty of hypnorum in the garden though. The confusing thing was the very dark wings,but she definitely had pollen baskets.
I have once seen a hypnorum male with the tawny brown covering the top half of the abdomen as well as the thorax, they seem more variable than some species.