Tomatoes Going Rotten at the Bottom

Hi folks,
I have four big, healthy tomato plants sitting on our balcony, and each one of them has about 30 (if not more) nice, plump tomatoes sitting on them.

All of the tomatoes are currently a beautiful deep green colour, and most of them are starting to go slightly translucent which I believe (this is only my second year growing them, so correct me if I'm wrong!) is the phase just before ripening.

However, three of them went red last week, and each and every time they did, the underside went black and rotten. It's always the underside. Pluck the red tomato, look underneath it, and it's gone black in a sort of 'parabola' form, from the bottom up.

Have they got a disease ? Is the death of all them inevitable? Since they're going 'translucent', would it be possible to pluck them now and ripen them on a windowsill?

Your advice would be gratefully received.

 

Just googled. Yep, that's exactly what it is.

I water the plants every second day. Too much ?

 

Thank you for both. I'll try to water them morning and night, but just give them a little less...

 

Hi,
They're in large (30 cm) pots out on the balcony, and I drilled drainage holes into each of the pots.

 

How do I know when it's needed ?