this is caused by letting your pot or grow bag not having enough water it is called blossom end rot keep your plants well watered, as the fruits swell they will need more water keep them well watered dont let your pots or grow bags dry out

That's probably what's called "blossom end rot". Not actually a disease, but a calcium deficiency in the fruit, usually caused by insufficient soil moisture at night. If they are growing in containers, I would water more often, and maybe add lime or gypsum to the soil. If they are growing in the ground, water more deeply, and put about 5 cm of coarse mulch around the plants, at least as far as the foliage spreads. Lime may be useful in the ground, too, if the soil is very acid.

Snap!

To help make sure the water/feed gets down to the roots I was given a tip on the allotments that seems to work brilliantly. When you first plant your toms, plant an empty plant pot next to each one (About a five to six inch tall pot is ideal) Then it's just a case of filling each pot with water and you know it's going directly to the roots!
When I pulled my plants last year the roots were starting to grow into the pots!

For any desert gardeners reading this, that only works where the soil or water isn't salty (containing sodium). watering from the bottom up like that tends to concentrate salt.