According to wikipedia via google..lol...:
Blue Milk is a drink in Star Wars. In Star Wars Episode II: Attack of the Clones and Star Wars Episode IV: A New Hope, blue milk is shown being served on the Lars moisture farm. Blue milk is, as its somewhat basic name suggests, a dairy drink either naturally or artificially colored blue. The protein-enhanced beverage is served on planets low in natural resources—especially water. Blue milk, coming from the teats of Banthas on Tattooine, is also rumored to be very nutritious and have certain medicinal properties.