We have the opposite problem in that we can't keep a pink hydrangea!
Any pink slowly turns blue. A friend bought me a beautiful lavender one too
and that turned blue

I would look for calcified Sea Weed and give a dressing of this working it
into the top couple of inches of the soil if you can,
failing this ground Limestone, something that will give you slow release of
lime.

--
David Hill
Abacus Nurseries
www.abacus-nurseries.co.uk