Random thoughts.
How many calves do you have and how many died and how far apart did they die?
Feed from feed store could be a problem, but could ask around and see if anyone else feeding it is having trouble.
Number one suspect would probably be the hay. Possible toxic plant or some other toxin in hay. Could ask your vet about testing it, though could be hard to find something if it is scattered in random bales. Is anyone else besides you and neighbor feeding your hay and are they having problems?
I would get some different hay, and feed that to at least half your herd and see if difference between groups on your hay or new hay.
I'm guessing since the blackleg spores are in soil, more likely to be infected during summer when grazing. But perhaps you could stir them into the hay, also. Flooding increases risk of blackleg - don't know if that has happened.
You might try taking temps of calves - higher early with blackleg. Also run your hands over the swellings and see if you feel crepitation, or crackling of gas pockets produced by the organism.