Blue hydrangeas

We have no fewer than four hydrangeas in the garden, two of them having grown into pretty large bushes. Although she has never said as much, I reckon that the hydrangea is probably one of Mrs Pensioner's favourite plants. I have noticed that, apart from clipping out the dead flower-heads at the appropriate time, she never cuts back those plants. They must be about the only ones in the garden that never succumb to the secateurs. But, much to Her Ladyship's disappointment, all four plants produce pink flowers. That is because our soil is very chalky and hydrangeas in chalky soil produce pink flowers. One of them was bought for its blue flowers and even though it was planted in ericaceous compost in a tub, the flowers, which were blue when we bought it, came out pink the next year. We have tried other ways of producing blue. Mrs BP heard that one could bring out the blue by burying iron in the soil so, when we planted one of the them, I had to trawl junk yards for a load of iron and dig a hole large enough to bury said metal beneath the hydrangea. But it didn't work. Madam was, therefore, absolutely over the moon when she found we had a large (very large) hydrangea in the courtyard of the house we bought in France - and it has blue flowers!