The answer should seem very obvious.
But first of all, if it was black leg, there should have been crackling under the hyde if you were to palpate the body befor opening it up
Black leg is a rapid gas producing disease which gets into the muscle and formes gas while turning the musclew tissue black,
An eight way vaccine has 1 more strain of a vaccine than a 7 way a 5 way has 5 strains etc. there are oodles of different vaccine products out there, you can visit with a local vet to tell you which vaccines cover the most common diseases in your region for your managment scheme