BER is mostly caused by too little calcium or erratic watering. It is not a fungus or other disease.

If the plant can't get enough water for the rapidly growing fruit, it seals off the end, and that's the "rot".

Start by improving your watering habits, mulch the plants to stabilize soil moisture, and see what happens.

Usually with cucurbits, the blossom end rot is caused by lack of pollination. Un-pollinated fruit will start shriveling at the end and eventually fall off the plant. This happens a lot early in the season before the pollinators are out and about. Best thing you can do is hand pollinate as soon as both male and female flowers are present and open.
If you do a search on 'hand pollination', there will be lots of posts with several techniques. I use a small paint brush if I have one handy, or I just cram the male flower into the female flower.