Courgettes rotten tips

It's exactly that - blossom end rot (BER).  With courgettes, it is often associated with the female blossoms not getting pollinated.

Try pollinating by hand. I use a small artist's paint brush to gather pollen from male blossoms (long thin stem, internal structure a single fuzzy column covered with pollen; pollen can also be gathered where it drops from the column into the flower).

Transfer the pollen with the brush onto the reproductive part of female flowers. Female flowers have a shorter stem, shaped like a miniature fruit. The interior reproductive structure is complex, looks sort of brain-like.  That structure is where you wan to deposit the pollen.

I did this all summer and have produced many courgettes. It is best done in the morning while the flowers are open and fresh, each flower is open for only a part of the day, usually morning.

Good luck!

I read somewhere that if mud gets splashed in tube flower while you water, this is what happens. I guess again it’s because it doesn’t get pollinated.

I have another courgette question. 
Why do some grow evenly all along their length, but others swell up from the plant end?

Cheers,  Tony.

I think that it works like this....some modern hybrids are self fertilising, so seed set is even along the fruit, the ones that grow bulbous at the end, are older varieties and only the seeds at the 'fat' end have been pollinated.
I think that's another snap SP 

Well... if they do go bad, I'd dispose of them.  If the fruit is just weird-looking but otherwise good, I eat them!