Your first sentence was the answer ;o)
Just kidding, these wouldn't be potted plants would they? Potted plants are notorious for BER. And yes too much, too little water could interfere with calcium uptake (the "real" culprit and most widely accepted cause of BER)

No, there are planted in the garden. So what is the solution? I hate to think all my hard work has been for nothing..

If you check the Tomato forum, there are some conversations about this going on. The general consensus seems to be that if you wait, it will go away; it's a temporary issue.

Yes, usually the first tomatoes are the BER ones, but as the plant's roots grow deeper then the moisture swings will not take as big of toll as to cause further BER. Hope they grow out of it for you pronto...

harriet,

blossom end rot caused by inconsistant watering and lack of calcium for the plant, the calcium uptake can be affected by the inconsistant watering. have heard that misxing some dolomite into a watering can of water and water the plants with that will help?

me i usually watch my watering regime and the problem goes away. you can still rescue some of the damaged fruit for use maybe in cooking?

len

Here is a link that might be useful: len's garden page