The blue flower colouration is a response to the level of aluminium available to the plant, and occurs naturally under acidic soil conditions. If your soil is not naturally acidic you can:

a) grow in a large pot using ericaceous compost
b) regularly apply aluminium sulphate to the roots of the plant
c) lower the pH of the soil in the area of the plant so that it can take up the aluminium that is already available in the soil.

Some soilless potting mixes don't have much aluminum in them, so the aluminum sulfate is the way to go there. Sulfur pastilles and/or composted sawdust will lower the pH on those planted in the ground.

Or you can water Sequestrene iron tonic around the roots twice a year. Available at the garden centre in sachets, mix in a watering can.

Maybe there's a different formula in the UK, but the Sequestrene we have here has no effect on soil pH. It makes iron available to plants in spite of a high pH.

Not sure of the formula here, but what I do know is it keeps blue hydrangeas blue rather than that nasty shade of lilacy pinky "I don't know what colour to be", Tugbrethil.

Well, whatever its mode of action, it sounds like something I need to try. Won't know 'til spring, though.