Blossom end rot is caused by calcium deficiency. Easiest solution for this is to just crush up egg shells and put them in and on top of the soil. I save my eggshells in a jar in the fridge for a few months prior to planting. It may be too late for this year but should work wonders for you in the future.

I had deformed cumquats from my tree until I started blending egg shells in the spice blender and watering them into the tree base.

Seconded, I would do this with my mom each year in her garden when I was a kid. Works 100% of the time, and it's a non-invasive way to address the issue.

Not always. Sometimes it's because it's too hot at night.