Manky courgettes

[farendwoman]

One of my Courgette “Midnight” plants is producing fruits that are going manky on the ends.
the other plant of the same variety is fine.
Any ideas?????

[ameno]

Your picture isn't displaying, so it's hard to say for certain, but two main possibilities occur to me.
If the fruits are going bad when still relatively small (no more than around 3 inches long) then it is almost certainly inadequate pollination. You should probably try hand-pollinating.
If it's happening to appreciably larger fruit then it could be blossom end rot, which is caused by calcium deficiency, usually itself caused by dry soil. But I don't think this is that likely, as courgettes aren't overly prone to blossom end rot, and if it were that I would expect it to be affecting all of your courgette plants, not just one.