Well, it WAS blue when I bought it, but has been pink ever since.
I have been reading about changing the colours of hydrangeas and apparently I have to control or alter the pH of the soil, which is easier if the plant is in a container. Suggestions include aluminium sulphate, organic matter, powdered limestone or wood ash.
It used to be in a container, but I planted it in the garden last year, so it may have to stay pink.
The lacecap hydrangea (hydrangea macrophylla normalis) has a rich blue flattened head, and the clusters of small flowers are surrounded by larger flowers on the outer ring.