If a flock is in plague proportions and gets startled on take-off, you could get a high kill rate. But wouldn't someone have noticed that big a flock of blackbirds?
I've seen a plague-sized flock when I was a kid - it was galahs (aka pink and grey cockatoo). I was a teenager driving with my parents to visit my brother who lived out past wheat country. It was harvest time, there were huge piles of grain which were being loaded into trucks. Some spills were being cleaned up by galahs, plus when trucks cornered on the road more grain was spilled. The fields were pink and grey, wall-to-wall birds. So was the road. As we drove though it didn't matter how slowly you went, you hit birds. Some of them were too full to take off. Others had no room to take off. Many collided in mid-air. The ground was littered with dead galahs.
I could believe fireworks could be responsible, but it would have to be a fairly dense flock of birds to begin with.
Marg