I've seen these black bumblebees before, and had always identified them as male field cuckoo bumblebees, not having found any other black bumbles to identify them as. But when I published these pictures, Roger @chardbirder kindly pointed out that cuckoo bumblebees do not have pollen sacs, which these bees clearly do.
He proposed that in stead of Bombus Campestris - the field cuckoo bumblebee these may actually be Bombus Ruderatus, the ruderal bumblebee, a far rarer species. These are actually the only TWO black bumblebees to be found.
This is not conclusive, as Roger said, ruderatus is a large species, and these guys didn't seem tht big to me. But the presence of a pollen sac is indisuputable.
I hope they are ruderal bumblebees, I just love the thought that our town park harbours something that is really quite special!
|A black bumble, with a pollen sac|