This is why:
They have Blossom End Rot. It looks like a ghastly disease, doesn't it? It's not a disease at all though, it's the effect of calcium deficiency in the fruit. This can often occur when the medium in which the plants are growing becomes too dry. In these conditions the plant finds it difficult to supply calcium to the fruits. If you want the full facts, the Royal Horticultural Society's website is a good place to begin. Here's a link.
Most of my tomato plants are growing in so-called "self-watering" containers, with reservoirs in their bases, and in any case I think I am more careful than most people about watering, so I'm surprised that my plants are affected. Maybe the damage was done 10 days ago when we had that really hot spell?
Once a tomato (fruit) is affected by BER there is nothing you can do to save it; all you can do is reduce the risk of further damage by making really sure that your plants always have enough moisture, even if this involves watering twice a day. I'm hoping that in my case the damage will be limited to just a few fruit. So far I have removed a couple of fruit from each of 3 of my 20 plants.
My self-watering pots have an arrangement whereby if the reservoir gets full the water drains out via an overflow. In order to stop this water going all over the place the pots are equipped with saucers, but significantly the excess water has no means of being re-absorbed and just sits in the saucers. Up till now I have been using the saucers, but I have decided to dispense with them, at least temporarily. What I will do is water the pots until I see water coming out through the overflow, at which point I will know that the reservoir is full. I will do this every couple of days.
My home-made pot-saucer arrangement is actually better. The base of the pot sits in a deep saucer (aka seed-tray without drainage holes) and therefore can re-absorb any excess water via capillary action.
Right, that's BER sorted (I hope). What can go wrong next??? Don't you DARE mention blight!