Most cases of blackleg I've seen started with a prostaglandin injection. I still recommend annual boosters for cows; the vaccine is so cheap I don't see a reason to cut corners with it.I used to think that cows that had been properly vaccinated as calves were good for life. But... I've seen cows with good calfhood vaccination histories die with classic blackleg lesions ... and almost all were in an estrus synch program. Seen enough of them, that we started boostering adult cows in our herd every 3 years or so... it's cheap insurance.
Not all cases will have crepitance (bubbles) that you can feel under skin or in deep muscle tissues. I've posted a bunch that had lesions only in the heart or diaphragm. Even if you cut into big muscle masses in the limbs, you could miss the diagnosis... most will have lesions in big muscle masses - with the characteristic dry, red-black appearance and separation of muscle fibers by gas bubbles, with a characteristic 'rancid butter' smell... but not all... if you don't look at heart, diaphragm, tongue, on those, you may miss it.