Hydrangeas are generally red, pink or white. This does depend on the breed. Some varieties will remain just pink, but for example the Hydrangea ‘Renate Steiniger’ can become a beautiful sky blue!
A blue hydrangea likes acidic soil. Acidic soil is mostly a sandy soil with a little lime in it. So watch the watering. If there is too much lime in your water, then it is better to give it rainwater. Collecting it yourself in a rain barrel or waterbutt is very useful!
Do not give the hydrangea any fertilizer in the months of March-July. The less manure, the bluer the flower. After the winter a little cow manure can be run through the top layer of the ground.
If the flowers are still not blue enough then consider alum. It is sometimes sold in chemists as an astringent for after shaving. Use 20-100 grams in (hot) water and pour it in mid-September near the plant. Your blue hydrangea blue colours will be beautiful!
Any tips you are given about using rusty nails or old pennies, we generally refer to the realm of fiction. If the soil is not acidic enough, this will do little or nothing out of it.