I looked up a book I have and found this;

Blue flowers on hydrangeas are obtained by growing pink varieties in a rather acid compost (pH 4 - 6) treated with alum.

Good results have been obtained with the following mixture
3 parts by bulk of acid loam. 1 part by bulk of oak leaf-mould.

To each 1 cwt. add 2½lb. of aluminium sulphate.

Pot plants in this mixture as soon as they come from the propagating frame.

Outdoor plants can be 'blued' by top dressing the soil in February with aluminium sulphate at approximately ¼lb. per stem, but it is practically impossible to get any result on highly alkaline soils.

Markedly acid soils produce blue flowers without treatment.

You mention peat & chicken droppings, peat I can understand, but I always assumed chicken dropping were highly alkaline, as is mushroom compost.

This is a preview from a section of my ammended website;

To make your soil more alkaline by 1 pH

apply 270grammes of Ground limestone per square metre (8oz per sq yd) to sandy soils

apply 550grammes of Ground limestone per square metre (16oz per sq yd) to loamy soils

apply 800grammes of Ground limestone per square metre (24oz per sq yd) to clay soils

Over-liming is difficult to correct, add less than the recommended amounts at first and readjust the following season after further tests.

To make your soil more acid by 1 pH apply 70grammes of Ammonium Sulphate per square metre (2 1/2oz per sq yd)

I hope this helps!