The blossom colour of a Bigleaf hydrangea (Hydrangea macrophylla), i.e. whether it blooms pink, red or blue, firstly depends on the variety. The blue colouring of a hydrangea blossom therefore does not work with white or purely red blossoming hydrangea varieties.
In the case of varieties with blue blossoms, however, it may occur that, instead of remaining blue, their blossom colour becomes increasingly reddish over the years. In order to prevent such blue blossoming hydrangeas from turning a dirty shade of pink, you must keep the acidity level of the soil at the site of the plant in the garden (the so-called pH value) acidic (pH 4.0 to 4.5). This is achieved by avoiding lime fertilisation on these hydrangea but also not using any other fertiliser containing lime.
This acidic soil must also contain sufficient plant-available aluminium. Ultimately, it is the aluminium absorbed which enables the plant to build up the blue pigmentation of the blossom.
Only when these two components are combined, acidic soil and sufficient plant-available aluminium, do the hydrangea blossoms consistently remain nice and blue.
Tip: When planting, provide the location for your blue blossoming hydrangea with plenty of peat or peat-replacement humus substances. Apply these again every year, due to the natural degradation of these soil-acidifying humus applications.