Campanula lactiflora colour shift!

My Campanula lactiflora bloomed beautifully last year and was the expected lovely shade of periwinkle blue. Hands down my favourite plant in the garden.

THIS year, it has grown and is lovely, but bloomed a pale pink. Not nice at all! And it all fell over in the rain!

How can I correct the colour for next year? Some kind of soil amendment, as one does with hydrangeas??