Blueberry contains an abundant amount of anthocyanins, the natural pigment that gives the red and purple color of the fruit. Anthocyanins have strong antioxidative effects which can help fight against free radicals, aging problems as well as cancer. However, this pigment is unstable and is easy to break down due to heat and oxygen once the structure of the fruit is destroyed. Because of this, it is normal that the color of your ice cream slightly changes (no longer stays bright purple but becomes darker) from the time that you make it to the time that you eat.