I've been told blue hydrangeas need an alkali soil (with a pH less than 4.5). Adding aluminiun sulphate seemns to be the way to do this if your soil is too acidic.
You can buy special pellets in most garden centres to change the colour from pink to blue. Good luck.
Waiglet....a pH of 4.5 is acid....extremely acid. Hydrangeas do prefer an acidic soil (peaty or clay soils) and this will indeed keep the flowers blue as will the addition of aluminium sulphate.....you can probably buy a "blueing compound" from the garden centre.
Hydrangeas also like pretty moist soils so try not to let them dry out in summer.