Some things to check:

First the pot, does it have drainage holes at the low points? If not-- drill holes. Is it the right size for the plant? If you have a dwarf windowbox then 2 gallons might be OK.

Second the Mix, does it drain without holding water too long? If not, then repot with appropriate draining mix.

Did you have a COMPLETE fertilizer in your initial potting mix including calcium? make sure you do.

Next; were you watering when the plant needed it, or on a predetermined schedule? Check moisture at root level with your finger, and look at the plant for wilt. Dont water unless the plant needs it.

Are you giving the plant a periodic fertilizer after the initial fertilizer is depleted? The watering of containers with fast draining mix will deplete fertilizer faster than in the ground plants.

Consider these questions and how many of these things need to be addressed. This is a a little late in the season for some locations, so some special things may have to be considered due to remaining growing time.

Of course one needs to assure the plant has good sunlite and right temperature.

Green Light Blossom End Rot spray is a organic calcium supplement to spray on tomato plants. I give mine a mist with it once per week and have not any problem with BER since I started using it.

You only need 3 oz mixed in gallon of water.

It is available at Lowes or on Amazon.

Here is a link that might be useful: Green Light Blossom-End Rot Control

I grew Window Box in containers a few years ago and the first several toms on each plant had BER. I've read paste/roma types are more prone to it. I just removed those fruit and as the season progressed the plants got past it. You may not get many ripe fruit due to starting late, but if the pots aren't too large and you end up with a lot of greenies, you can take the plants inside to extend the season when frost is approaching.

It's unlikely that the BER is because of an actual shortage of Ca in the medium - it's probably because, for whatever reason, those fruits received an inadequate supply of Ca as the fruits were forming. Since it involves the first fruits, it's very possible the deformation occurred very early in the plant's life - before transplanting. If you bought the plants as sets (in cell packs), it's likely that something cultural caused the issue even before you acquired the plants, but things like over-watering, applying Epsom salts to the plants/soil, high humidity and cloudy days during fruit set - in short, anything that disrupts or slows the nutrient stream, can cause BER.

Al

This is my favorite read about BER.

Here is a link that might be useful: Click here for BER article

Good info - thanks. In case not everyone reads it, it points to cultural influences as the primary, rather than an actual shortage of Ca, in most cases.

Al

In my experience Roma tomatoes are more BER prone than others.

This year for the first time I have a problem with BER. It is also the first time I have planted Burpee Big Boy tomatoes. My other tomatoes are not affected. The simple solution is to be selective by going back to my Better Boy next year. Al

I too, had some BER issues this year, but only on a tomato named Rutgers.

Al

I had BER at a lot of my first blooming tomatoes this year, I did nothing an all the other one that came in were fine. Also you can cut the BER part off, the tomato is edible.