They are normally blue on acid type soil, Try mulching with Ericaceous compost and feeding now and again with azalea type plant food.

Good luck.

Give them a shot of sequestrene iron tonic

they need acid soil!

They prefer it slightly acidic, Great, but you can get round it by administering sequestrene yearly if you want really blue flowers - they will grow quite happily in neutral soil too, just the flower colour will be that strange sort of mix between pinky lilac