Blue flowers are relatively rare in nature and they are mostly pollinated by bees. It is thought that bees see blue and blue purple as one colour and may perceive it as green. It's thought that the colour that human eyesight perceives as green, as in leaves, is simply a neutral background of dull yellow grey to insect eyes and green flowers show up vividly against this background.
The blue in the flowers is produced by anthrocyanins which vary in colour according to the pH of the sap. Acidic sap produces a more purple colour and alkaline sap produces blue