Try this:
sarahs mum said:

acid. not acid enough.

<<If your hydrangeas are pink (because your soil is alkaline) and you want them to be blue, you need to make the soil acidic by increasing the presence of aluminum. You can achieve this by adding amendments to you your soil like pine needles, compost, coffee grounds, and aluminum sulfate that help make the soil acidic over time. Keep in mind that changing the pH of your soil is a gradual process and will not result in deep blue blooms overnight. It can take up to a year for the color change to happen.