Very dark Bombus terrestris queen

Yesterday I saw what I thought was the first Bombus lapidarius in my garden - I'd seen a couple of others at the edge of the wood, but non in the garden. Anyway she was bigger, or rather fatter than normal, so I went to have a look. She was huge! But her tail was not the usual red, but a dirty brown, and just the very tip. Then I had a better look. She was actually a Bombus terrestris queen. The yellow stripe on the thorax was completely missing, but this is not so unusual as it can be very faint in terrestris, or dirty and covered in dust and soil. But usually the abdominal yellow band is there even if it is a darker dirty yellow, but this one had none. What she did have as she moved into the sun was a sort of sheen of dark ginger/blackish hairs where the yellow abdominal band would be. Unfortunately I didn't have my camera so I couldn't photograph her. It wasn't pollen, or anything else because when she moved into the sun I could get a good look at the hairs with the sun shining through them, and she was clean.

Has anyone else seen this?

I've never seen such a dark terrestris in all the years I've been watching bumblebees. Perhaps she was just a mutant. She was large, and otherwise seemed perfectly normal feeding off an ornamental currant bush.