Blue flowers

I took this image of these very pretty blue flowers growing in my rockery just before the heavens opened with heavy rain here this afternoon but I have no idea what they are called.

I planted these in my rockery about 7 years ago and as well as the blue flowers they have purple coloured leaves and are excellent at keeping weeds at bay - well apart from the one you can see growing amongst them (horses tail it think)

If anyone know the name of the blue flowers I would live to know.