Definitely have one tested. They can definitely tell you if it is blackleg. You say "young calves".....how old were they? "Ususally" blackleg affects calves in the 6 months of age group......but can also go up to two years. It usually affects the best ones. 'Can even affect young thrifty calves as young as 6 weeks. It usually occurs in the summer and fall months, seldom in winter. Get all of your calves up and vacinnate for blackleg. Also, give them a penicillin shot at the same time. This will help with any that might have acute symptoms. Unfortunately, once a calf gets the symptoms, he is dead with 12-48 hours.
I lost about a 700 pound heifer this fall that had been vacinnated for blackleg. I first thought "blackleg", but it turned out to be acorn poisoning.