Hi Julia....I think this is called end rot...due to sporadic watering and not regular. Are they inside a greenhouse, or outdoor plants?

Hi Janey is quite right, but this can also be caused by lack of calcium. Try adding one table spoon of epson salts to a gallon of water and spray the plants. If the plants are well established another trick is to crush a few Tums and work them in to the soil around the plant. It is not contagious and won't spread to other plants.

not enough water for a while then a rush of water gives you blossom end rot angels advice will do the trick

Thanks to those who answered my query, sadly the tomato plants are no more!! They were in the greenhouse, will ditch these. Is it too late to try again with some bought plants rather that seedlings?