By Fructibus – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=63719100

Blossom End Rot is a disorder caused by the lack of calcium needed by plant roots by way of proper and consistent watering. It appears on the bottom of fruit (blossom end) as a small blemish that increases in size over time into a blackish, sunken lesion (canker). It is possible more cankers can appear on a single fruit. They lesions are usually hidden by the plant’s foliage so it is possible thy could be missed from a cursory glance. Blossom end can occur in any vegetables in the Nightshade and Cucurbitaceae such as tomatoes, peppers, cucumbers and squash. The blemished fruit should be discarded, but with time and the proper care, the plants can start producing again.

By Siltloam – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10897152

To prevent blossom end rot, make sure you:

Check your soil pH level before planting (levels should be around 6.5).

Add 2-3 layers of organic mulch immediately after planting. At that pH level, calcium and other nutrients are more readily available for plant use.

Water your plants regularly during hot, dry periods. The plants need at least one inch of water a week all at once (drip or soaker irrigation is a no-no) to deeply penetrate the soil and saturate the roots thoroughly.

Use compost, seaweed/kelp emulsion or fish emulsion to fertilize your plants. However, be careful not to over fertilize.