Sounds like blossom end rot - caused by insufficient and irregular watering,disabling the plant from taking up enough calcium to aid fruit growth/ripening

Yes that sounds the very thing. Happened to me one year when I asked a neighbour to water them when we were on holiday. It's annoying.

Most of the tomatoes on my indoor Roma plant had blossom end rot too but none at all on the outdoor ones.?
I'm now told that adding calcium tablets or epsom salts to the compost is a good idea but haven't tried it.

A local gardener informs me that PlAnts growing small tomatoes never suffer from blossom end rot