In article , Franz Heymann
writes


Hydrangeas need a soil with a substanrial amout of a certain Iron ion
in order to produce blue flowers. Try using Squestrene or a similar
product. Ask your neighborhood garden centre what bluing agent for
hydrangeas he stocks.


And in order to make the link between Franz's answer and mine regarding
the pH - it is more difficult for plants to take up iron from an
alkaline soil, hence the blue flowers in acid soil and pink in alkaline.

--
Kay
"Do not insult the crocodile until you have crossed the river"