Posted: 13/07/2013 at 07:05
It does sound like BER. Here it is in an advanced state:
It's not just irregular watering that can cause it though it can be a factor. Keeping the soil moist isn't a factor. BER is caused by plant stress stifling the plant's ability to distribute calcium to the fruit via its internal mechanisms. Strong winds can cause it, seriously fluctuating temps, overfertilising, etc. Anything that will physically stress the plant.
In addition, some varieties - the plum-shaped ones, like San Marzano, etc - are simply genetically prone to BER. No one knows why.
One or more tomatoes on a plant suffering BER doesn't necessarily mean the plant's entire production will suffer. If whatever is causing the plant stress is addressed, later toms will be fine.