Ours were due to normal things like stones and feathers, then there was the piece of string that wrapped around the tongue that eventually cut the tongue off and killed the hen.
Sorry to hear of the loss, it is never easy to lose a pet. Blackhead doesn't usually run rampant through a flock although it is always best to treat all the birds in a pen if needing to treat one bird. Controlling the worms is key to controlling BH but the cure for BH is metronidazole. I have...
I have had young birds contract BH without the presence of any worms, also BH is not detectable on a fecal slide, it takes a different test. Concerning your Strike pellets, could the bird be eating other things and not be eating the pellets? Good luck at the vet and keep us in the loop.