The blue colour is often the sign of chilling and very cold nights. Was this possible even in a conservatory given the poor weather at that time? It might also be a symptom of overwatering if you were giving them lots of water at a time when there was little sunshine and poor light.
At least you sound as if you have escaped blight which affected many more people this year, both outside and under cover, and even here in this part of France.

Thank you for your two suggestions, both of which are distinct possibilities. They are close to the door, which I hook open for most of the day unless tipping with rain, and I may well have been a bit gung ho with the old watering can. Lesson learnt! Thanks again.