If you suspect it's blackleg, get a conformation - FAST!!!!!
One of the members of our community pasture did not vaccinate his calves for blackleg at branding time some years back.
HUGE MISTAKE.
Outbreak was noticed when calves started dropping like flies...in the hottest part of summer. Not just his calves, but due to the extremely conatagious nature of the disease, also calves that had been vaccinated a month before turn-out. Apparenty, the calves in herds vaccinated closer to turn-out time had more protection from the anti-bodies they had developed, as they had a lower mortality rate.
There aren't enough words to describe the Hell that the members went through to comb out every head of stock from a bush mountain pasture (muskeg, river breaks, logging slash choked with regrowth, coulees and sheer cliffs thrown in occasionally for good measure) 30 miles long and 8 wide, in the dog days of summer. 2000+ head. Every single surviving animal was revaccinated. Within four days of the realisation of a blackleg outbreak. Difficult is the understatement of the year if you are familiar with how cattle "bush-up" in intense heat.
That particular gentleman was responsible for not only the death of 2/3 of his own calf crop, but the other members herds experienced an average loss of 10 per cent. It was the worst fall money wise for these people (pre-BSE in Canada, that is). All for the want of saving a few bucks at branding time.
I am not trying to cause a nervous breakdown, but for your peace of mind, if a vet suspects blackleg, get it confirmed or denied by a vet. Yesterday!
I'll be keeping my fingers crossed for you. Take care.
ps. Vicki sounds right on the money about the abomasal ulcer. Lost a cracking good calf today to one. Three weeks old. Hate it when that happens, but c'est la vie.