The short answer
A dark, sunken patch at the blossom end is often blossom-end rot, a disorder linked to calcium reaching developing fruit unevenly. Inconsistent watering is a common trigger.
Why this happens
The soil may already contain enough calcium, but root stress and fluctuating moisture can interfere with transport. The damaged part of an existing fruit will not heal; improved care protects later fruit.
What to do
- Keep soil moisture steady without waterlogging.
- Mulch outdoor soil to moderate drying.
- Avoid excessive nitrogen feeding and root disturbance.
- Use a soil test before adding calcium or changing pH.
Keep in mind
Eggshells are not a quick remedy. Other fruit diseases can look similar, especially if lesions appear away from the blossom end.