To keep a blue hydrangea, aluminum sulfate may be added to the soil around the hydrangeas. You can buy this to water in. This is easier to achieve in container grown plants

Or a shot of sequestrene iron tonic applied round the roots.

The above advice is only relevant if your Hydrangea is planted into neutral or alkaline soil. Should stay blue of its own accord if planted into acid soil.

Thats what I was told too Sid, but it didnt work on mine which were in pots in Ericaceous Compost ?

That's odd Drc726 - were the flowers blue when you bought the plant?

Yes they were blue but only for that flowering Sid

How curious....well, if you ever find out what's happening there, do tell me wont you!!

I got fed up and recycled them

If you have hard water, that can neutralise the ericaceous compost. Our water here is about as hard as it can get--we just settle for purple! : )

tug's right - ericaceous compost becomes non ericaceous after a while, specially if tap water is used. Which is why there's always a need for sequestrene for blue hydrangeas in tubs, lol;-))

Our water is very hard and being in pots they got a lot of tap water thanks now I know.

That explains it I suppose.