It has always been said that the acidity of the soil determines the color of the standard blue hydrangea...
but horticulturalists say it is the aluminium in the soil that makes blooms blue...acidity just makes this more available to the plant. To make the aluminum available to the plant, the pH of the soil should be low (5.2-5.5).
from one of my gardens - Jan Johnsen
If you want blue blooms - ensure that the soil is ACIDIC...supplement the fertilizer with any of the following:
aluminium sulphate (1 tablespoon per gallon of water ),
ground up orange or grapefruit peels.
from another of my gardens, needs some 'bluing' - Jan Johnsen
If you use fast acting aluminum sulphate, apply it as soon as flower buds are evident in the new shoots in the spring, about 6 weeks before flower maturity. Important: Water plants well in advance of application and put solution on cautiously, as too much can burn the roots. You can apply this every so often throughout the growing season
Also you may choose to use a fertilizer low in phosphorus and high in potassium to help produce a good blue color (25/5/30 is good. Potassium is the last number). Avoid Bone meal when trying to produce blue.
If you live in areas with very alkaline soil or if the hydrangeas are planted next to a concrete wall or if you have alkaline water then you may not get blue hydrangeas even with all your ministrations....
note: WHITE HYDRANGEAS can NOT be changed to pink or blue.
Gloster furniture and Hydrangeas