How to prevent Roma tomato rot
This is blossom end rot (BER), a common tomato disorder. It is caused by a calcium deficiency - not because the soil lacks calcium, but because the tomato plant cannot take in the calcium. There are several reasons that this happens: uneven watering and other weather related phenomena, root disturbance and over-fertilizing. Also, some varieties are more prone to get BER than others ('Roma' is notorious for BER). Tomatoes grown in pots have trouble with it, possibly because of uneven watering. Read here:
The non-rot portion of the tomato is still perfectly edible. In the future, pay attention to watering and cultivating around the plants, and don't over fertilize. I'd also recommend switching to another paste tomato variety. There are varieties that allegedly aren't prone to get BER. Unfortunately, nurseries often offer 'Roma' as their only choice for paste tomato.
Thanks for the info - I will see if I can find an alternate tomato variety for next year's planting.