What is Blossom End Rot?
Blossom end rot is recognisable by the very end of the squash or courgette turning brown/black and rotting as it grows. This happens when the plant is short of calcium, but more often than not they will be short of calcium because there is not enough water in the soil for them to draw the calcium up though the roots. (It’s unlikely that your soil will actually be deficient in calcium).
How to prevent it
Blossom end rot is is most often caused by periods of underwatering, so make sure you water your tomatoes regularly keeping the soil consistently moist.
The good news is that Blossom end rot is not a contagious disease - so if one courgette on a plant gets it, then it doesn’t mean that the other ones will. Just try to keep your plants well watered so that the soil is consistently moist and the new fruits should be ok.
A fortnightly liquid feed once flowers and fruits start to form on the plant will also help.