The main reason for `Black leg` on toms is that they have been allowed to get chilled, possibly if you bought them from a garden centre. It`s too late to cure now but in future look before you buy and always wrap well and keep away from draughts until well established in the greenhouse.
Also people wonder why when they get home from a garden centre or store with a nice new plant after a day or two it snuffs it, the reason why is in the above answer. Always make sure a plant is kept out of cold winds by putting another carrier bag over it before leaving the store.
Some people think I`m being nosey when I see it as they leave stores and advise them to cover their plant in the cold, but I`m betting they wish they did next day.
Hope that helps.