Leave the flower heads on till the spring, then cut back to a healthy bud and wait, there will be a new growth of flowers to come and if your soil is on the acid side they will be blue again.

If it used to be blue and turned pink you need to acidify the soil and one way is to water it with ammonium sulphate. This is often sold as a proprietry package for bueing them but plain ammonium sulphate is cheaper.

If it is still in a pot you can repot using ericaceous compost and only water it with rain water.

The reason for leaving the flowers on over the winter is that it protects the young buds from frost damage.

If it is in the ground and your soil is alkaline it is unlikely to look a perfect blue even with the ammonium sulphate. GCs sell blue ones in areas they fine we'll know they are not going to be easy to keep that colour.

In that case put it in a tub if its small enough to move.

I was told if you put rusty nails in the soil around the base they will turn blue.
Not sure if it’s true

It isn't Peter, one of those 'old wives tales'.

I think whoever told you that Peter was getting confused with the effects of iron deficiency in some acid loving plants like camellias and rhododendrons. That causes yellowing leaves not flower colour change and doesn't apply here as MG says. (Dunno if nails would help there either actually as its the pH of the soild that prevents absorption of iron rather than the lack of iron in the soil))

I doubt nails would work for acid loving plants with iron deficiency either Stera...

...or people either...