Various studies on flavonoid pigments related to the color development of blue flowers and their biosynthetic genes have been conducted for over 100 years. Today, molecular breeding of blue flowers has become possible via the genetic manipulation of the pigment biosynthetic pathway present in some flowers. Recently, blue transgenic chrysanthemums have been successfully generated by synthesizing artificial anthocyanins that interact with endogenous flavonoids. In this review, I will describe and discuss the recent advances in blue-flower color breeding and the blue coloration mechanisms that help generate blue transgenic chrysanthemums.