Some of my medium size fruit varieties have just started to set and it is at this stage that I am most concerned about Blossom End Rot.
BER is the result of a lack of calcium as the fruit is forming and affects the underside of medium and large fruit varieties leaving a dark leathery patch. When the fruit has formed it is too late to add calcium.
The thing is, it's not that there isn't enough calcium in the soil (usually), it mostly happens because roots aren't working to full capacity owing to dry areas in the soil or cold temperatures.
I took this slightly out of focus picture of a plumb variety two seasons ago and every tomato on the plant was affected by BER ... rather disappointing considering the time and effort that went into getting the plant to this stage (the camera was more interested in the fence!).
As soon as the pea-like fruit appear I make sure that I give the soil or compost a good watering and a foliar feed with a food that contains calcium or trace elements at this stage is, in my opinion, a good idea.