Katy is absolutely right - there are many gardeners who absolutely swear by the rusty nail trick. If you think of it, the bluest of blue/purple hydrangeas are grown in Cornwall where the underlying soil contains copper and other minerals.
There is also the notion that the colour of a hydrangea flower is an indicator of the pH levels of the garden soil - pink blooms appear in chalk soils, and blue in more acid soils. So, if you have the sort of London clay that I have in my garden, I think I might need more than a handful of nails to reverse this!
But it is certainly true that you can control the soil acid-alkaline levels better in a pot, but then that presents other problems in that you have to take great care to keep the plant well watered - it is entirely at your mercy on the amount it can get, and pots have a very nasty trick of drying out at a moment's notice!
MANY THANKS FOR THAT INFORMATION ON THE HYDRANGEA, I WILL CERTAINLY TRY IT AND LET YOU KNOW. BY THE WAY I THINK SHOOT IS ABSOLUTELY GREAT, I AM A NEW MEMBER AND AM STILL LEARNING HOW TO PUT MY GARDEN DESIGN TOGETHER.
Adding aluminium sulphate to the soil encourages them to stay blue. This is found in hydragea blueing compounds, which you should be able to find in most garden centres. This is particularly effective in container grown hydrangeas, so you should witness great success with your plant!
There is also an old tradition that placing a few rusty nails in the soil keeps hydrangeas blue - haven't tried this myself but many swear by it!