Hi AR Farmer,
Blackleg is a specific disease caused by a bacteria called Clostridium chauvoei and usually affects grazing cattle although it can be found in contaminated feed. It can be fatal and this is why we usually vaccinate our girls against it.
Foot rot however covers a variety of complaints, everything from 'Fouls' to 'Interdigital Fibroma' to 'Digital Dermatitis', all caused by different bacterium and therefore requiring a different treatment regime.
Blackleg usually affects the whole leg [hence it's name] while Foot rot affects just the hoof, however if left untreated most Foot rot bacteria can spread and infect the inner muscle/bone and then require partial amputation of the infected claw.
Fingers crossed you never encounter any of these complaints, they all cause lameness and a drop in milk production due to the pain and for this reason need a fast reaction to treatment when observed.