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From memory.... Hydrangeas like iron in the soil to give a rich colour to the flowers

If you apply lime to the soil; a regular practice in the old days for clay type soils, it will neutralise the iron needed for Hydrangeas Rhododendrons etc, lack of iron produces pale washed out flowers; sometimes blue in colour

When I was making wrought iron work for the garden several years ago I always collected the iron filings metal dust etc and put it round the base of Hydrangea and Rhody
This may have helped to give the strong colour