Premature blueberries, the white or green ones, won't ripen after being plucked from the bush. But the ripened ones that are blue/purple will continue ripening. That's why they're only picking the ripe ones (text box 5 ) so it should probably be
@herz It's pretty clear, by now, the story doesn't take place in our world, not even in just a slight variation of it. In that sense I feel like it doesn't matter that much if they are blueberries, not bilberries. In the previous chapter Theoporon said he wanted to bring down a polar bear (a white bear), but couldn't find one, so he had to settle for a brown bear (by the looks of it). I don't think he would have gone looking for a polar bear if the world was like ours and that place is in North Europe. Boars also don't live that deep in the north.